Because they are silly and chasing numbers. I can stream 4K TV at home on my Apple TV 4K and 65" TV with 60Mb/s Xfinity service. Why these people think they need faster than 100Mb/s LTE on their phones is beyond me. If they are getting slow LTE speeds now because of overcrowding or poorly located towers etc, it isn't going to get much better with 5G. It might get faster at first when only a few people have phones working on it, but it will quickly get overcrowded just the same as others upgrade their phones.
I personally believe they Cellular providers should be beefing up their LTE service, adding new towers and bandwidth, covering more area with faster LTE speeds etc. Rather than chasing new specs so they can brag about being the first or fastest in limited locations.