It’s pretty much the same as anything else: you’re paying for more and better precision, more vibration damping, more aesthetics, more exotic materials, that kind of thing.

In this case, putting an AT-LP60 against a, say, a Pro-Ject Debut, the Debut has the more solid body, the more solid platter, a carbon tonearm, comes with a better cartridge, and moves the power supply off-board to the wall. All of those improvements move it into a different echelon of performance, which will cause you fewer headaches over time.

It’s relatively easy to get something to play a record, but it takes some finesse to play a record well, and that costs some money.

The quality of the pickup will differ. A cheap one won't necessarily have the quality of the expensive ones, using a steel needle Vs a diamond needle or the like. There's also the problem of cheaper units having arms that aren't balanced properly. And incorrectly balanced arm can wear your vinyl, which is a problem if you're playing "originals". If you're playing some mass produced modern stuff that you can replace you may not care.

My cheapy unit has a "heavy" head putting around 7grams of weight on the records.

That may not be an issue for you.

You'll be shocked at how good modern amplifiers are. The amp will not be the key to your system unless it is very cheap. Speakers and even room treatments will have a much higher impact