Carriers have far more customers than addresses. With carrier-grade NAT, a phone gets a private address inside the carrier's network, and many phones share one public address when they reach the internet.
For a website, this means one mobile address stands for a crowd. Blocking it would turn away every real customer behind it, so sites are reluctant to. That reluctance is the whole reason mobile proxies are accepted where other kinds are challenged.
Read next: Mobile proxies: price and details