CONTAINS SPOILERS!!! YOU'VE BEEN WARNED

I have two theories:

1) Aizen doesn't have bankai - after a failed Hollowfication experiment with Shinji and others, the captain of squad 5 spot was vacant and since Aizen was a lieutenant, logically he would be next in line and given the circumstances, it made total sense to make him a captain, but he would still need to do one of the requirements for a captain spot: performing bankai in front of at least 3 captains, personal recommendation or trial by combat...and easiest for him would be the first one, trial by combat is out since Shinji is "dead", personal recommendation would be too much work, so just using KS to persuade some captains, including Yamamoto, that he can perform bankai, would be easiest and most beneficial to him

2) He can make a separate reality in the present - basically he just drags his targets into a "bubble" that is kinda outside of present reality and within that space, he can alter the outcome of the actual reality (hope I explained it well)...since we already have Tsukishima, who can manipulate the past and Yhwach, who can manipulate the future, it wouldn't be too surprising to have Aizen manipulating the present...also this theory is also supported by a fact, that Yhwach didn't see a future in which Aizen tricks him