That’s kinda like asking why does any company make and sell any tool rather than just using the tool to make money? Why does Milwaukee sell power tools to contractors rather than keeping those tools to themselves to profit from contracting work?
Because that company does not mint (supposedly) thinking machines - the lack of humans at a price point that is feasible is often the reason - it would be more profitable to sell the tooling because its impossible to maintain all the layers of specialization within one company.
It's the same for software companies too, they do much more than just producing code, it wouldn't be feasible for Anthropic to do all that other stuff too.
But LLMs are supposed to replace all white collar jobs (according to the frontier labs), not just software engineers. They should just have agents run the other aspects. Or maybe they are selling FUD?
Eh but if Milwaukee was to then invent a power tool that it claimed could soon do the contractors' whole job... OPs question would correspondingly exist there too.
A student of history might note that power tools do indeed allow a worker to replace multiple workers not equipped with power tools.
Not the same though, equivalent would be a tool, rented from Milwaukee, that does all the work with minimal input from contractor .