That’s just not true at all but ok.
OK, lets dig in the data to put away our biases. The nba.com gives the following list 7-footers who played>30 games at 10 MPG+ in 23/24: Wembanyama, Porzingis, Brook Lopez, Holmgren, Lively, Kornet, Ayton, Gobert, Hartenstein, Zubac, Poeltl, Embiid, Nurkic, Hayes, KAT, Markkanen, Mitchel Robinson, Queta, Mo Bamba, Kessler, Aldama, Nick Richards, 22 overall.
Now, according to basketballreference.com, Markkanen is the last, 13 out of 13 listed in DBPM (they have a high minute cutoff for these rankings). Also, Markkanen is listed there as 167 out of 185 total, he is surrounded mostly by offensive guards out there.
Now, lets look at the defensive estimated plus-minus at dunksandthrees.com: Markkanen is 20 out 22, with only Kornet and Nick Richards placing below him. Such defensive powerhouses as Lamelo, Duncan Robinson and the current version of Joe Ingles scored above him on D-EPM.
In short, different advanced defensive metrics list Markkanen as one of the very worst 7-footers getting consistent minutes. And the eye test is certainly confirming that.