That's a subjective statement.
After I played Bannerlord for a while, and was disappointed, I asked myself the same thing.
I asked myself wether Warband (native) really was so much better than Bannerlord or if that was just my imagination from all the time I played with mods.
I started another game in Warband (native) and at first I thought; hell, Warband sure has some issues.
It's graphics are horrible, the randomized battle maps with their extremely unrealistic steep mountains were a pain in the butt and a lot of the issues I was complaining about in Bannerlord are present in Warband as well (mostly AI related).
But after an hour the magic reappeared.
There is far less of a grind. If you know what to do you can get going relatively quickly compared to Bannerlord and if you play intelligently you can even with just some dozen men swing the fate of wars (or at least try to). Also whilst the game had it's problems, at least it was playable. No broken sieges, less ridiculus steam rolling and most importantly of all: a degree of depth and immersion Bannerlord does not posess. Laugh about feasts and the like all you want. But they had their place and were important to worldbuilding.
Use the tools available to skip the grind. I mean they literally just dumped most of the worst grind in the game just now and a while ago opened up an easy way to get into holding territory. You get up ready and quick fast in bannerlord.
No broken sieges? Mate, sieges would bug out plenty.
What depth and immersion? Feasts? Is that depth and immersion? A mechanic I have put on hard ignore for YEARS. I guess the companions had more personality because they weren't procedurally generated and had things to say during the game. I do miss them, though I barely remember the vanilla ones I have fond memories on the pendor ones (the mod I played the most)
What steam rolling? The stuff they fixed?
The UI in warband is strict worse, there's no subjectivity in this one. The battles are worse, unless I guess you are some wierdo who hates having more options and formations and less weird terrain. And, shocking, without mods warband diplomacy and kingdom management gave you LESS to do than what is in bannerlord right now.
There are not many things vanilla warband did better than current bannerlord. Most of your issues are pretty dated.