I don’t want to sit here defending 5e but 80% of the complaints I hear about always seem to boil down to “why isn’t the system creative for me?!”. It’s a lot of people self-limiting and then being mad.

You can instantly create a harder, thoughtful encounter by simply introducing more enemies than just one they can beat on, and/or by doing WHAT THE BOOK SAYS and get the players used to multiple encounters per day so they need to manage their resources. My DM wanted to make fights harder and I simply mentioned that a stronger enemy is cool and all but what would be better is making us have to make choices. I was a stupid accurate fighter and focused on range, and while feats and stuff made me a dangerous close-quarters fighter I was also the only one who could reliably down other ranged enemies. We played up to level 13 in that campaign and there were a lot of fights that were pretty stressful and fun. We even had a tournament arc and that was wild.

Your inability to create complex encounters is not the fault of the system, especially when the system literally tells you how to make it work and you ignore what’s in the book. But, of course, not reading the material is pretty standard procedure for D&D players.