This is sorta the beginners philosophy question. There are plenty of answers, it's not the "gotcha" it appears to be. Those answers unroll into all sorts of branching other conversations but they exist.
Maybe it's because free will exists.
Maybe there's a greater purpose for what we call "evil" that results in more good.
Maybe it's a definitional thing, where "evil" to us is always going to be the most-evil existent thing so if existing evils were gone "evil" would still exist but it would consist of aggressive kitten licks or something. So "evil" can't not exist, but it's not because God can't get rid of what we call "evil" now.