There's likely a statement in Play Services ToS for vendors to do all things possible to prevent bootloader unlock/relock flow from happening - reasoning from the fact that yellow AVB state is non-existent outside Pixel devices. Maybe it goes as far as for SoC vendors, as well. So far, outside of Huawei, no top tier hardware vendor ever decided that denying Play functionality to their users would be profitable - also all Mediatek based devices are basically licensed by mediatek afair, so there's no chance of, say, Vivo/Realme suddenly deciding to ditch Play and do bootloader relocking.

Also the possibility of postmarket devices running non-bloated OS is a loss for a vendor since it both reduces the appeal of whatever next "+1% cpu +1% battery" lineup update (and its a bad idea to sell 200k "good device model 1" rather than 100k "bad device model 1" and "bad device model 2", because PR/stocks/whatever) and increases the possibility of having users dissatisfied with the brand name because battery/flash degradation is still a thing.