Why is last hiting so important mechanic if...

You sound like someone who doesn't play rumble or mordekaiser.

In particular, I'm pretty sure rumble is known for having particular difficulties last hitting compared to a number of champs. It's a little better now that flamespitter does full damage to minions, but even so.

Rumble used to be one of the hardest champions to last-hit effectively with due to his melee-range, clunky autoattack animation, low damage-per-tick on Flamespitter against minions and the low tick rate on Flamespitter.

While Flamespitter no longer deals reduced damage to minions and the tick rate was increased(IIRC), he is still not a simple champion to last-hit with against competent opponents.

Rumble's auto animation is pretty terrible compared to most champs And flamespitter isn't good for CSing with early on

Dude, do u even play this game. Cs is more important than kills at times. You can keep up with a stupid enemy laner who gets kills in other lanes through it. It is where you get gold from. Also, those champs are hard to last hit with. Also, you saying that means you suck worse than a 9 year old.

Mordekaiser is easy to farm with Max W like you are supposed to, then farming is the easiest thing ever, or if you don't want that, max E. but W makes farming freelo.

as rumble, you gota be careful when using Q to farm because it's easy to shove the lane in and get the enemy jungler's attention on you due to said shoving/pushing. morde miiiight have a little less trouble with this if hes using Q to farm. old morde E hit HARD, it was his main damage ability and it shoved WAY hard, but was effective against some champs like nasus cuz it was ranged super heavy harass. and easy farm.

point being that if you're going to use champs like them that can easily shove a lane, you better be ready to set up some wards and hope enemy jungler isn't super mobile or you'll be setting a huge "free gold here" sign on your head.

As a Rumble main I can confirm that farming is really hard, but most of Rumble's power in lane comes from keeping the wave pushed and bullying your opponent when they try to CS.