Just because that instruction never appears (as such) in scripture.
Though it is also quite possible that the reason why the two oldest sons were bringing their own fire was because they had disobeyed by letting the fire go out (Leviticus 6:9-13 which had come from the presence of the Lord a few verses before (Leviticus 9:24). So fire came out from the Lord again and consumed them (Leviticus 10:2). Why else would they be bringing fire in their incense censers, if it had not gone out on the altar?
It's also quite possible that they were schnockered at the inaugural worship ceremony (prompting God's otherwise-irrelevant observation in 10:8-11), violating after one year the 40-year fast from bread and wine that God had imposed so that Israel would depend upon His providence (Deuteronomy 29).
Or possibly they committed the most heinous act of all: serving as priests before God without understanding or recognizing what He wanted of them, even after seeing Him on the mountain, seated above a transparent sapphire pavement (Exodus 24:1-11). Imagine that. Seeing God Himself. Eating and drinking in His presence. Hearing His voice. Rehearsing His commands for this crucial worship assembly ... but not knowing Him; not recognizing the holiness He wanted them to display.
I guess those possibilities would not be expedient to the argument of the Nadabites and Abihuettes.
Their argument is that God will utterly destroy anyone who disobeys Him by doing something that is not specifically authorized (especially in worship). That's how they picture God: keeping His will secret from His servants, at the edge of His throne watching for the moment that He can cast fire down from heaven to consume them at their first and slightest infraction of it.
And that, sadly, reminds me of the way that the steward entrusted with one talent envisioned his master: solely wrathful, greedy and vengeful. He was afraid to do anything with the talent that he hadn't been specifically authorized to do. As Matthew re-tells the story, the master didn't leave specific instructions. So the third servant did the no-risk, nothing-ventured-nothing-gained thing to do: he buried it (Matthew 25:14-30).
(Of course, he hadn't been authorized to bury it, either.)
The first two stewards took some risk, transacted some business, put themselves out there to honor their master's house and to increase the esteem of others at its assets - and his wisdom in choosing and investing in stewards for it.
They were generously rewarded.
Why? They knew their master and what He wanted. They knew what they needed to do to please him, and make his business thrive. (And what would sharpen their skills and acumen to serve him even more effectively, later on.) He didn't have to tell them what he wanted. They just knew.
The third servant did not know his master, maligned his nature to his face and then hoped he would obtain mercy for his cowardice.
In this story - in this life - God is our Master. His business is the gospel of His Son, Jesus, the Christ. It is the ultimate treasure: the salvation of our souls; the restoration of our hearts to His.
He knows how much He can entrust to each of us, because He gave us our trading talents as well as the talents to be traded.
Don't we know what He really wants?
Shouldn't we put ourselves out there when we're transacting gospel business for the chance at gaining the maximum return on His investment?
Shouldn't it be that way every day of the week we're in business, instead of just one day (which is all it takes to bury something)?
Let's face it: We're not specifically authorized to sit or stand motionless singing in four-part a cappella harmony with hymn books in our hands, following only one song leader with a pitch pipe or a tuning fork, in a nice big comfortable church-owned building, either. So if we decide to start forbidding how hearts gifted by God want to worship Him, where do we draw the lines that scripture doesn't?
At one day a week? At one day a week, plus maybe a Wednesday night? At one person speaking, rather than two or three? At one cup? At projected lyrics and/or music? At clapping? At a praise team leading? At accompanying instruments? Which instruments? At whatever I think is decent and orderly? At what my brother or sister thinks is decent and orderly? Do we draw the line at what does or doesn't praise God, because we think we know Him so well through His silence?
Here's the picture I have of God, and I get it straight out of scripture:
- One who wants us to express His praise, His wonder, His love and His power fully and with all our hearts (and be blessed by doing so!), whether we are gathered in worship, or worshiping by serving, or serving by sharing, or sharing by teaching - giving up what we want to do in favor of what He wants for us to do every day of our lives.
- One who reveals through this testing what His good, pleasing and perfect will actually is.
- One who knows that we we blessed by doing so, built up in faith, empowered by His Holy Spirit, transformed into the image of His Son.
- One who wants us to put ourselves out there - way out there! - and take some risk. Love deeply. Live purely. Transact some gospel. Give it up. Give it all. Don't hold back (Romans 12).
Because that's what He has done and does for us.
That's what I find Jesus doing, and later, those who followed Him.
And if we picture our God and model our vision of Him before others as being miserly and stingy and secretive and vindictive like the third servant did, He will indeed become the God we fear ... but do not love and trust.