Vendors wrote those standards though, not the operators. They are complex because it’s a defensive moat against competition imho
My take is that telecom operators forcefully had to evolve from Morse and copper to IP and radio, hacking stuff left and right every step of the way.
[1] https://en.wikipedia.org/wiki/IP_Multimedia_Subsystem#Archit...
In contrast there were the Qualcomm CMDA standards which were all proprietary and in-house, which had some upsides, but in the end carriers preferred the open standard with vendor competition.
There's a book called "GSM and UMTS: The Creation of Global Mobile Communication" about the history which is really interesting.