The problem is that a lot of people wanna play a MMO like a solo game and do ALL content available. 99% of time you will be declined because there are applicants with better resume than you (ilvl, class, IO) and the keyholder doesnt have a reason to invite you over them. It is not about toxicity.
You have 4 easy ways to fix this problem:
1- Play a tank/healer, If you are competent, people will fight over you in their Group.
2- Join a guild and talk to people.
3- Use your own key, then you can invite whoever you want.
4- Go play a solo game, It probably will suit you better.
The only thing Blizzard could do to help alleviate this problem is to create some incentives to tank/healers (a bag of gold, extra item for the tank/healer). Or maybe give a bag of gold to players who bring low ilvl people on their runs (boosters would love It).
But I belive any player can fix this issue, to themselves, with some effort. And stop act like strangers on a virtual world own you something.