Well it’s more than that. Are encounters varied? We have a very skilled player who found a red dragon and fire giant are not too worried about fireballs, scorching ray etc.
Now that was an unforced error perhaps. But if folks are not pretty experienced, this will happen in various ways of the DM is throwing variety.
My friend was vexed in another session: “I can only prepare so much! This has been frustrating!”
Yeas wizards are powerful but more and varied encounters are the DMs province. I was eldritch smiting left and right and kept my fireball in my pocket.
Wizards are a bigger problem at higher levels in the hands of people who know how to shape the game.
At lower levels and with less experienced players the problem is minimized. I suspect that they did not try to counter this level of player of higher skill levels at higher levels and also think they may not have 100% realized it.
Even with experienced players it happens. Whether it was the best tactic or not, one of the players in my wife's campaign decided to focus on fire based spells. Fights against anything with resistance (until he took the feat) meant they contributed little. When we hit immunity, they were just SOL. The high level wizard (we played to 20th) was basically down to cantrips outside of 1 or 2 spells.
But more to the point, if you get more than 2-3 encounters per long rest against a wide variety of opponents and tactics and in my experience is that wizards are not all that. But we already have a thread for that.