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And not everyone has time or energy to read all books once, let alone "at least a couple of times", and there are many player-facing books some DMs never read unless they need to look up a spell or class, etc. Time is a resource not everyone has to put into RPGs as much as they'd like - if you do, that's amazing, but many don't.
But other than that could be plot or story reasons, including that the standard races don't fit their view of the setting - such as DnD 5e having drow when previous editions had them as deeply seated in the evil monster race, which is where some DMs leave them.
As mentioned, there is also power creep - that affects most games eventually - as companies need to make their newer books stronger to entice players to buy them.
I would rarely say it is ever a fear, usually a preference not to have X in their game, whether it overpowers everything else or doesn't fit their idea of what the game is.
This message was last edited by the user at 09:07, Sat 05 Dec 2020.