The main factor is there are several prominent places to buy cards that people base their value off of
They can be fun for casual players or streamers who like the occasional big splashy moment, but they're not so good for players that want the game to feel competitive
The Kasla, the Broken Halo display commander is printed with the foil-etched treatment on a thicker cardstockgreat for showing off your commander but not legal in sanctioned Commander play
What MTG Cardstock Actually Is (and why its called Corona) Magic cards are not printed on normal card or random cardboard
The best time is before a set releases via a pre-order at MSRP from your local game store
Not even just the holos, but commons and uncommons like Eevee, Exeggutor, Butterfree, Rhydon, Weepinbell, etc