what is BTC doing wrong ? why dont they fill the whole block first and then calculate the merkle tree

I think I have the basic intuition, but please correct me if I'm missing something. I have my blockchain midsem tomorrow and, naturally, I only started actually appreciating how all of this works like 2 hours ago instead of studying whole year.

Anyways so if miners get the 3.125 btc + transactions fees wouldn't they always have an incentive to fill the block as much as possible to get more fee?

what is btc.com doing wrong do they want to make these semi useless blocks because doing so they will reach the new point when reward gets halved faster(it will happen in a year) or are they incorrectly accessing mempool . only the btc is mining these half filled blocks almost everytime foundary usa or antpool is the one to win the mining race.
this is the site if anyways is wondering : https://mempool.space/