Because in Hero System you can only put so many points into a single power, and if the GM says, "No single power of yours can be built with more than 60 points," then that's it. Plus, in Hero system, all the power effects are designed around this, and any power built up to 60 points, whatever it is, is more or less, equal to each other in scale. Not so in MnM.
I don't think that's exactly true of Hero; 60 pts in RKA (4d6 RKA, unless it's 2.5d6 AP RKA) is not the same as, say, 60 pts in Desolidification (Desolid, No END), or 60 pts in Damage Reduction (and I've completely forgotten the point structure of the power, and I'm pretty sure it's changed every edition).
Now, 60 pts in RKA should be about the same as 60 pts in EB -- but it's not even the same as 60 pts in a HtH normal damage attack.
Of course, Hero has Damage Classes as an additional limit (at least through 5th ed.; I haven't read 6e) for attacks, which helps avoid having a 4d6 RKA vs. a 20d6 powerpunch. It also recommends setting limits on defensive values, too, as I recall.
I understand why MnM went the way of Power Level, to mimic the strucutral class foundation of the d20 system...and that is what it does. Each Power level limits how many ranks you can have in different things, just like a class level does.
It doesn't limit everything, though. Primarily, all it limits are combat stats (attack, damage, saves) and skill totals (via skill ranks and ability bonuses). The combat stuff is more-or-less the same sort of limitations as Hero suggests GMs impose.
The limits on skills are there to keep people from doing obnoxious things, IMHO (like buying Diplomacy +100 and trying to argue that you can talk anyone into being your friend), and because skills can become combat traits (inherent to the skill, like Bluff, or via feat, like Acrobatics + Acrobatic Bluff).
Also, PL is strictly a limit for PCs; for NPCs, PL is descriptive (you make up an NPC, then figure out what PL he is -- just like you make up an NPC in Hero, then you know how many active points he has in his powers, DCs, etc.), and for PCs it's just a cap. In D&D, level tells you something about how powerful a PC must be -- because every level gives attack bonus, hit points, saves, etc., so there's a minimum value of combat power. In M&M, I can say, "make up 170 pt PL11 PCs", and a player can make up a character that's actually about PL 6, thanks to low combat values & skills. Spent all those points on wacky mobility powers and every skill, maybe.
From my POV, M&M is as much a class-and-level game as Hero is. They're just different games, so of course they feel different.