What makes you think they didn't. BG3 has been in development for years. They tested and iterated the game long before it went to EA.
And the result was a system which uses half the rules from 5e and half the rules from a homebrew, and those two rule sets don't work well together.
What does it matter that Larian tested and iterated the game long before it went to EA? It was not a perfect game then, and it's not one now. That's the reason Larian is doing EA. To get feedback from other people outside their own bubble.
Developers can think something is great which players end up hating (The D:OS 2 armor system). Developers can lose confidence that something is good as is and change things which didn't need changing (Adding a Source point cost to the Bless spell in D:OS 2).
