Check-in time is typically 3pm or 4pm and is listed on the webpage for each property. That's what they're advertising and promising. Anything beyond that is what you'll have to negotiate yourself.

How full was the hotel on the night before? How many Plat+ were staying? A hotel can't turn over a room to you until the previous guest is out and housekeeping and if necessary maintenance, has been in there. If you've got a bunch of Plat+ occupying rooms to 4pm, all of those rooms are blocked from anyone checking in early.

I've only needed/asked for early check-in twice in the past 12 months. One was granted, one was denied. On the denial, the hotel was legitimately sold out the night prior. I did reach out to the hotel the night before and the front desk said they'd try to fit me in, but that they were sold out and I did confirm through other sources that they were indeed completely booked.