Depends on a variety of factors.

• Sometimes, they are just sticklers for policy. Sometimes they're just unhelpful.
• Other times, particularly at peak periods, they may literally not have any rooms available that are vacant and cleaned.
• You have to take into account the fact that they may also not have "real-time" information on the status of cleaned rooms. I don't know that many hotels have cleaning staff send in real-time updates upon completing the cleaning of each room.
• Even if rooms were cleaned, perhaps there is no room of the type listed on your reservation.

I routinely (like every week) take a 6am flight to NYC and arrive at my Manhattan hotel by 930am (give or take). This Marriott has mobile check-in and my room is almost always ready by the time I arrive - usually in time for breakfast in the lounge, also. A couple of times, they've asked if it's ok that I get a room with double beds instead of a king, and that's the last thing I care about, so I've always accepted. Then again, I'm also getting to know some of the staff, so there is that.

I've also successfully checked in early several times at the Hilton a couple blocks away.

Of course, I write this as I sit in the lobby of a boutique hotel in London where I was unable to check in early and, as I need to head into the office in about an hour, walked across the street to have my suit and dress shirt pressed - they should be ready in about 15 minutes. London hotels are SO inflexible...