Their competition is the enterprise hardware divisions of HP, Dell, and IBM. SuperMicro makes reasonably good quality, lower-cost server equipment. They are, IMO, a pretty good value if you don't want high-end support from the hardware vendor.
ASRock has also pushed into some of Supermicro's traditional product segments via the "ASRock Rack" brand. I have no idea how big that business is, though.
Gigabyte and Zotac also comes to mind, resemble Asrock.
There's a bunch of other kit too, but https://servethehome.com reviews a bunch of the various rack systems.
Example of some late January posts. Albeit none are of the "fits lots of GPU" sort that is helping propel Supermicro, but these folks all have those offerings too,
HPE, Dell, and IBM are glorified CDW-business model salespeople. Megacorps have no use for that when they can engage the source and get their own custom gear.
