Waiting is inconvenient to buyers and to explain it away, Xiaomi provide some silly reasons such as saying they are trying to thwart sideloading of apps and custom ROMs by resellers, that does nefarious things like steal data... it's a silly excuse, and the wait time inconveniences buyers who want to unlock and install custom ROMs.

Edit: I suspect Xiaomi do this to keep you using MIUI as long as possible because they make money from showing ads in various parts of the system, and also sync your data back to their cloud which probably is resold in some derived form to advertisers. In my opinion, MIUI sucks. MIUI vs. LineageOS proved it is the phone OS (software) which influences how much a person likes a phone, rather than the hardware.

They've recently increased the wait time from 2 weeks to 2 months which had buyers complaining loudly on forums. So for their newest device (the Pocophone) they reduced the wait time to about 3 days. But I think the long wait times still exist for other phone models.

The wait time depends on Xiaomi's mood-of-the-day. They keep fluctuating it.