Something that's happening frequently at my table recently is that the players are making choices deliberately to waste my preparation.
If they see something that obviously took some preparation that they can skip, they'll skip it on purpose and then laugh at ruining my plans. If they learn that I've done more preparation work for one choice than for another, then they'll take the option that wastes more of my preparation and then laugh about it. (They'll do this even if it's just because if they take that option, I need more material prepared, not that I prefer that option.)
(This was last session.) They are traveling through a desert when they spot a static tornado/giant dust devil a few hundred feet ahead of them. They approach and see that the tornado thing is swirling inside an ancient amphitheatre and that there's a music being played inside. The sand seems to swirl around the amphitheatre along the rhythm of the music.
Then the players go like "Oh, that seems curious, I bet you want to trap us inside that! Let's go guys! I ignore the tornado and continue walking on our way. Sorry DM, not this time." Then I'm like
"... Okay... You continue on your way..." Puts notes related to that aside
"We did it again guys!"
Players start laughing.
(The rough version of what would have happened if they'd investigated: They would have found a Simulacrum of a very famous NPC that's playing the music inside the amphitheatre, after a test she would teach the bard a stronger version of Mold Earth and maybe tell them somethings about the desert if they asked her.)
An example where there were multiple paths that they could choose to get to a place:
When they were trying to get to a village at the top of a mountain, they could hike the mountain and invade the village through the front gates, or try to sneak through some caves that would lead them directly to where they wanted. The group likes the stealth approach and I had prepared for that — I had maps for the cave and stuff. They choose to hike just because I had prepared the caves that much.
Some things to consider:
I never gave them a reason to think that unusual things are “the DM's traps”. I have never pushed the "you activated my trap card" button, like locking the players inside a trap room with monsters or anything like that.
PC deaths are quite rare on my table, and when they do happen, the bard quickly fixes that with revivify.
They are mostly, if not always, fully rested when they find “side quest” things like that.
“Side quest” content is pretty much always related to the main quest in some way. This might give them some info on something they didn't know about, a new magic items, some ability/spell that gives them another option later on and things like that.
The players know this. Our sessions are short so I've told them that we don't have much time for completely unrelated content to the main quest. The players want the story to move on, so almost everything is related.
Later on when they should/could use the information/power/whatever they get confused/frustrated that they can't go that way/don't quite fully understands what's happening.
There's never only one right answer/way to the objective, but there sure are optimal/more rewarding ones.
The only thing I can think of for why the players are avoiding side quests is that a few weeks ago, they were doing one of these side quests with one player that was an oathbreaker paladin. At one point there were 3 of the 5 players in a meeting with a few lords of the city. The paladin started insulting the Lords, and even used his channel divinity to frighten them. When they demanded that he leave he attacked them. The other 2 players followed his lead and the 3 of them died.
But they realized that that was stupid and not my fault. That's the only thing remotely like having a bad experience with a side quest.
So, they're skipping through the content that I prepared for the session, on purpose, for some quick laughs about ruining my preparation. Skipping this stuff will frustrate them later on, and I lost my time preparing for something that won't see the light of the day.
In the end, everyone loses.
I don't have that much time to prepare for multiple paths, so I usually focus on one, having only mental notes for the others. Skipping through what I'm mostly prepared for usually hurts the session as a whole.
I know that "Extra" and "Sidequest" means "hey, this is not MANDATORY", but if they don't have much reason to skip it, should I still just play along with 'OK"?
Is this just another "talk to your players situation"? I'm somewhat a new DM, so this might just be a part of it and I don't know.
I just wanna leave a disclaimer here.
I like my players, I really do. They are overall awesome and dedicated players, but it became some sort of trope or joke for them to skip things like this. It may be related to the case I talked about, it may not, it's not like they do this every single time, but they do it 2 out of 3 times without much reason. So, I just wanted to say that they are amazing players and that this is the only thing that I wanted to try to "fix".