I don’t think it has anything to do with laziness. Endless scrolling has been around for a long time and is really easy to implement.
They do this for one reason - because on average, this method costs less in data served to the average user than something like endless scrolling. They serve up the most popular comments to you thinking that is what the majority will want. Then they put a button to load more because only a small percentage actually use it, and that saves them money.
They don’t load the comments in order because they don’t want you to click the button, so making the results of the button inconvenient will detract more people from using it. It’s all about trying to subtly convince you to trust that the comments that they served up originally are the only important ones so that you don’t even bother to click the see more link. Enshitification at its finest.