One of the first house rules that I often put into my campaigns is “The information in the Monster Manual represents a collection of common belief and superstion concerning particular creatures which may or may not exist in the campaign world. Any fact stated therein may or may not be accurate. The more players rely on such knowledge without prior experience of the creatures in question, the more likely they are to be catastrophically wrong.”

Creating a homebrew world is a lot more work than using a published setting. But if you do it well, it can also be a lot more rewarding – if you enjoy that sort of thing! For others, who don’t, a published setting is the perfect answer – but their campaigns will always be handicapped to some extent by the problems that you mention, at least at first.

I have to include that final caveat because from the moment play actually starts, the players should start reshaping the campaign. The very history of play evolves the campaign world from a common foundation – to see how much it does so, simply invite a new group (that doesn’t know the world as it has evolved) to ‘enjoy’ a one-off game in the world. Havoc will ensue, and you’ll spend half the game explaining how things came to be the way they are…