The DSLR represents the end-point of an unbelievable period of hardware improvements in cameras, they contain a ton of amazing mechanisms some of which just exist to make the "SLR" feature possible. The big camera manufacturers learneed a lot over the past few decades and the hardware is all there because customers demanded it.

However, once LCD screens got good enough, the SLR aspect became a burden and if you look, most mirrorless pro cameras look very similar to their SLR counterparts.