I disagree - it's not cowardice to want a complete adventure as a base. The whole point of buying an adventure is that it gives you a complete story from the start instead of having to make up your own. Coming up with reasons that monsters are somewhere might be a fun little writing exercise for some, but it should just be a blog post, not a product people are paying money to run their groups through.
It might be cowardice to not change or expand on the adventure as written to customise it to your group, but even then it's a group thing - 5e in particular has lots of groups who approach the game with the same attitude they would skyrim or diablo, and their character is just a means for them to interact with the world. Changing the story for those groups doesn't actually add anything to their experience.
I broadly agree about ignorance being a problem, but I still don't think it's cowardice to not want to do things you haven't been taught how to, and particularly not with 5e - you don't learn how to run 5e by playing 5e, you learn by playing other systems then coming to 5e because that's what you can get players for. You play something like 2e and OSR or 3.5/PF1 and run 5e with the mechanics from those games in mind. There isn't a mechanical base that rulings can be built off in 5e, so it's not cowardly for new DMs to not want to have to come up with mechanics from scratch.