Because anyone who honestly believes that ‘clumping up’ is a prerequisite of Kael being picked clearly doesn’t understand how great a zoner he is, how solid his wave-clear is and how much influence he can bring with good positioning, even when doing minimal damage and/or fighting scattered targets. There is a reason why Kael has been consistently popular in almost every meta during competitive play. “Bunching Up” is an enemy mistake/concern - and isn’t always unavoidable. Of course, a good Kael would look for such opportunities, but they don’t sit around whining and doing nothing just because the enemy aren’t clumped in a Q-worthy 5man bundle. It’s not like any good team would be in such a position very often anyway.
As for Mal’Ganis, he is prone to kiting and control, with a kit that is entirely predictable, entirely interruptible (bar one ultimate - if he even gets to cast it), and doesn’t pack half as much punch as other options. Yes, he’s a solid dueller, bully and laner - but to call him OP would require being a QM player who constantly gets randomized teams that have absolutely no answer for him, which applies to multiple heroes in HotS, or any randomized mode in any MOBA for that matter.
It’s not uncommon for Mal to go unbanned or even unpicked by versed teams in ranked modes at the moment. Anyone with “instalock Mal” mentality will very likely encourage an instant anti-healing+CC comp that will laugh at his shenanigans if the team are even remotely capable of drafting appropriately. The problem is, even in ranked, a lot of people don’t give the drafting process the respect it deserves, in which case it is the fault of themselves, not the heroes they face.
As far as I’m concerned, Mal falls into that sub-category I call “just annoying” - but again, he will transcend annoying into monstrous if last-picked vs suitable comps, or you wade into Quick Match and get absolutely no CC or counter-play, no different to how people complain(ed) about Genji, Tracer and various other heroes who relish total freedom during team-fight chaos.