It's mostly an attitude held by more casual players who don't want to spend money buying specific cards to play along with the current metagame. They see netdecking as either paying to win, which in some sense it is, in Magic: the Gathering at least, or just 'cheating' when someone assembles a deck from a list they found online.

People who play magic more competitively definitely do not have this attitude and spend a lot of time experimenting with the meta, and reading about it and the deck archetypes for the format(s) they play in. Including tournament winning deck-lists.