Ask HN: Why do registrars force you to use their nameservers first?

For 'consumer' registrars, it's probably because 95% of their customers are small non-tech businesses who just want a website and don't know what DNS is, etc.

Nowadays, name server changes for well-maintained TLDs usually propagate within an hour or so based on my experience.

I just did one and changed it immediately and before querying the domain and it was OK.

However I have seen it take longer, and it might be because of caching by my ISP or something.

They don't. Good ones let you choose at registration time. Mine lets you pick a default in the first place of your own..

Perhaps they have been "parking" the domain previously, very common amongst some large registrars