IR bottom heaters are usually not strong enough for reballing. They're for boards that are hard to solder, because there's a lot of copper or a heatsink for example.
The bottom heater preheats the whole board, not to soldering temperatures but enough to make soldering a lot easier.
Those Nvidia cards used in mining and AI need reballs regularly, also ps4 south bridge often falls off, also intel sockets sometimes need reball, also you can upgrade ram on your phone and Nintendo switch
Oh for sure. I'm not knocking those that can do it, just that my regular soldering skills are shit enough that I'd probably be hesitant to reball something more complicated even with the right gear :-)