> you are not forced to use them, which makes them non-rent-seeking by definition.

That's not how it works. Those two things aren't mutually exclusive. Plenty of businesses engage in rent seeking without having a captive (by most definitions) audience. All that's required is a very modest barrier (ex network effect, non-zero switching cost, etc) and a sufficiently large audience.

Rent seeking isn't even mutually exclusive with adding value. A business can do both simultaneously by virtue of being able to multitask. Most businesses offer more than a single product or service after all.

The real value Steam "provides" are the network effects. That's rent seeking.