There was a brief window in time when you had to go out of your way to buy an Intel laptop "without" a TPM (even Macs had them for a time, even if Apple never made use of them). The Trusted Computing Group failed to capitalize on that timeframe by providing both a "reason" and decent solutions to that problem.
There's a lot of reasons why that was, if I've been drinking I'd happily go into many of them.
On the mobile side, I agree, it's a hodgepodge. Apple has their secure enclave (which doesn't quite act like a TPM, even though it theoretically could), and there exist vendors who could theoretically include a TEE in their phones (right now they're almost entirely limited to special "government-specific" use cases).
And I'm ignoring Samsung's solution (which is basically snake oil).
Intel's SGX would be great, provided that the industry suddenly switches to X86 for mobile (which I don't think is going to happen).
The mobile industry is way too fragmented from a hardware perspective for any type of trusted computing platform to achieve even a modicum of install base. That might change in the future, but I wouldn't bet on it.