Because the people building them aren't the people operating them, and there's a real tension in the objectives and goals between those parties.
Further - how do we balance out that the people building them have given themselves extra power (no restrictions, private models, special access to prompts and system prompts, unencrypted reasoning traces, etc) when operating the LLM themselves.
---
Even if the tool is good, it's a bad bet to build a thing on top of tooling with these limitations.
This is the same problem we have with mobile software stores and app distribution. Theoretically the store serves a purpose and can provide value, but practically the main goal seems to always devolve into enriching the owner at the expense of everyone else.
Even if we accept that LLMs should be limited (and I'm not actually sure I do) - the legal standards around what/how/when those limitations are used need to be VERY stringent.
Technology conglomerates are decidedly not "white knights" with a motto of "do no evil". They are self-serving rat-bastards. That should be the lens any power they wield is viewed.
The framing is absolutely not "anthropic is being careful with humanity". It's "assume the devil has control of those companies - how bad is the result of them having this power?"