You’re remarking on a development time number. Like, the amount of time it’ll take to create your account.
In whichever case, that doesn't mean we can't compose them into a performant product as the OP suggests, it just means you run them as daemons so you can amortize the startup cost across many invocations. This isn't specific to containers--we do the same thing for web servers, databases, virtual machines, physical machines etc. Anything with a startup cost that you don't want to pay each time.