I'll give a very simple example. I did a few SWE interviews in 2020, and several companies did the initial screen over the phone, and the on-site over Zoom.
In both cases it was a remote interview. There was no reason not to do both over Zoom. The only reason was that the previous process was a phone interview and then an in-person onsite, and they realized they had to replace the in-person on-site with Zoom, but they didn't think to replace the phone screen. If you started from scratch it makes no sense though.
In this case, the whole origin of the Leetcode interview is "we're going to hire the smartest people in the world.". You can dispute whether that was true back in 2009 but it was certainly part of Google / Facebook's messaging. Now, in 2024, I think it has morphed much closer to a standardized test, and even if people might begrudgingly admit that, there's still the cultural baggage remaining. If a company used a third-party service, they'd be admitted they're hiring standardized candidates rather than the smartest people in the world. Which might be an "unknown known" - things that everybody knows but nobody is allowed to admit.