Combat is weird. If I were your opponent and had played a Nurse Gargantuar in lane 1, your damage would register, killing me, before my heal triggered, even though my zombie attacked did damage first. Is that fair? Would you like to argue that I should heal and live?
Here’s another scenario, thanks to Monument: I play a ... let’s say Moonwalker on a Monument, on Heights. Moonwalker is supposed to get a +2/+2 buff. Instead, the bonus attack triggers first and triggers your super block. You Weed Whack my Moonwalker, and it dies before the buff resolves.
Okay, that’s not a combat thing, exactly. But it is a weird thing. PvZ has a lot of weird little resolution order rules that are unlike any other game I’ve ever played. I say learn the weirdness, and learn to live with it. (Because the plants are all going to say “yes! let’s do damage at the same time!” and the zombies are all going to say, “wait a minute! that’s our thing!”)
Edit: I’m guessing the Devs wanted to eliminate the potential for draws. Having zombies “go first” makes sense from a flavor perspective b/c zombies are dumb and aggressive. It may have also been a balance thing, but probably very few games are decided by striketheough/strikethrough (at least by percentage). The scenario above ^^ with the Nurse probably doesn’t heal b/c if “going first” triggered abilities, you could never get rid of a zombie with deadly. That doesn’t make me any less sour for losing games that way, of course.