In a collecting field that sometimes appears to thrive on its lack of logic (“seriously. you collect New Kids on the Block and Norwegian Death Metal?”), how we file our records would appear to be one over which there is no division.
Go into a record store. Have you ever visited one where the stock was not standing upright, with either the cover or the spine facing out? (Unless, of course, you stumble upon the store's storage area where records are considered to be in less-than-decent condition.) Search online for an old record carrying case. Have you ever found one where the handle is affixed to the widest surface, as opposed to one of the narrow ones? Even in the old days of Edison players and trumpeted gramophones, record racks demanded the records be filed standing up.
The reasons are simple and manifold. It prevents records from warping. It allows for better airflow for the records, and reduces moisture and temperature build-up. It reduces the risk of bent sleeves and ring wear. It helps keep the spine intact, so you can still read what is written on it. And it prevents people from inadvertently using your record collection as a place mat.
Laying them flat in a stack, on the other hand, achieves completely the opposite. Not every warped record, after all, was subjected to excess heat. Sometimes, it was simply unlucky enough to be two-thirds or so of the way down a high (and heavy) stack of albums. most of which, incidentally, will have a discolored LP-shape/sized circle embedded into the sleeve.
An individual album is not especially heavy; a pound or two depending upon the vinyl weight and the packaging. But even a couple of dozen - a four inch high stack - exerts more pressure than a single LP was built to withstand, and when you remember that, no matter how flat the surface looks, the LPs below it will all have their own “ups and downs,” basic physics does the rest.
Plus, as you will see when you go back to that record store. It’s an awful lot easier to find what you’re looking for when everything’s standing upright.