My semi educated guess is because different kinds of people sit on the two different standards boards, so you get siloed specs with the cross sharing of some good ideas. You could ask a similar question about HDMI vs thunderbolt.

The real reason why 5G and Wifi are separate is because one is licensed spectrum, and the other is unlicensed spectrum.

I'm not talking about legal differences- laws can always be changed- but rather how bandwidth is allocated. The biggest benefits of cellular comes from:

  Verizon owns/controls channel
        ↓
  gNB decides:
  "You transmit here."
  "You transmit here."
  "You get 20 MHz."
  "You get these time slots."

Wifi needs to follow listen-before-talk rules:

  Everybody shares unlicensed spectrum.
  
  AP: "Is anyone transmitting?"
   ↓
  wait
   ↓
  transmit

So at the end of the day, even if you take all the fancy expensive 5G hardware and software algos and shove them into the wifi standard, you still won't get the benefits of 5G.

5g-NRU allows what you are speaking of, but imo still needs quite developments for actually being deployed in bands with interferance (i.e. WiFi).

How would I connect to my own network? Wifi and cellular solve different problems.