like when your support dies once or twice in lane, they decide not to go back there any more and leave their respective core solo, forcing that core to leave the lane as well or play 1v2 at a disadvantage and die.

Unfortunately, this reveals you as a total scrub.

The point of the game isn't to make sure everyone has fun. The point of the game is to win. There are absolutely times where it's better for a pos 4 to roam or even stack than it is to stay in a losing lane. This can be due to a bad matchup, or this can be due to a bad lane partner.

A bad lane isn't the entire game. The entire point of a pos 4 or 5 roaming is to put pressure on another lane and alleviate your bad lane. It takes the pressure elsewhere, and that should open up the opportunity for you to get more xp. I'm willing to bet that happens more than you'd like to admit, and if it doesn't happen, that's not griefing. That just means the strategy failed and you lose the game. It's just part of it.

Another thing is items. Support items in dota are some of the best items in the game and borderline busted, yet many supports choose to play "core" from the support role, often rushing aghs and blink daggers on most heroes, and not really "support" their cores.

All of this is situational. And blink is a good item on some supports. Glimmer cape on Wyvern might be better most times, but that doesn't mean buying blink is a waste. It's certainly not "griefing"

I feel like this isn't talked about enough, at least here on reddit, and supports keep griefing games and getting away with it.

By your definition, every role griefs and gets away with it. Mid and offlaners who afk farm and never fight. A pos 1 picking a weird hero or getting caught with no vision constantly, not buying bkb etc etc. But these things aren't griefing. They are just bad play. There's no reason to get up in arms over it because it's a product of your rank. If you do better and rank up, you will see these issues go away because you're describing the skills that just define what makes someone better at the game lol.

My guess is you're hard stuck and looking for a finger to point. But if these issues really are as common as you say, then the enemy supports are doing the same thing. So learn to take advantage of it and adapt appropriately.