Well, the entire game is 5v5, and the nature of the game is that a single underskilled, trolling, or AFK teammate will stack the odds overwhelmingly against you. I'm personally never toxic, but I sympathize that keeping the competitive spirit alive while also being so adversely affected by bad teammates can lead to flaring tempers.
By contrast, look at WoW. In a 10v10, 15v15, or larger battleground, a single bad player has much less impact on your winning chances (especially since killing them doesn't empower your enemies). Plus, when you take a break from competing, the game offers PvE and solo content to cool off with. Toxicity easily still happens, but that style of game does a lot less to encourage it.