After watching on YouTube Viva La Dirt League D&D's GM who is thousands of times better than me*'s GM** I conclude these are some of the reasons it happens:
Players might be new, while the GM is the one who is expected to care the most, know the rules, be passionate.
Metagaming is important. Before starting the campaign do sit together and discuss where on the Story Mode*Mode***-Open World spectrum do your friends want to be. All is fun, except for mismatched expectations.
A good GM tailors the world to the characters (subtly). To their backstory, to their abilities. An even better GM asks the players to do it!
- Describe your melee attack.
- What is important to your character right now?
- How does your character look, how do they talk, how do they walk?
Reward assertiveness (the guy in the video does it extremely well). You want to try something unorthodox? It will probably work!
* - I wanted to advertise the yt channel as little as possible but a more experienced user decided more important is naming it
** who is thousands of times better than me; however the group still exhibits DM-first dynamics
*** - a la Call of Duty — a movie with some combat