Dota 2 has a lot of great qualities. The lack of a surrender option isn't one of them.
Rival MOBA League of Legends lets teams surrender after 20 minutes, roughly the halfway point for matches. Valve hasn't shown any desire to incorporate a similar feature into Dota 2. Director Erik Johnson explained the decision in an interview a couple years back (via Reddit):
One of the things we never want to lose is the amazing comeback, like the game we just played - both sides probably would have surrendered in that game at certain points. That would have been a robbery of fun.
This is a compelling arguments. Any MOBA veteran has seen their share of comebacks and knows they feel incredible. There's nothing better than hanging onto a game by your fingernails and then clawing your way to a victory. Even when you're on the losing end of these types of matches, you have to appreciate the roller coaster of emotions.
Let's be real, though: some bad games start bad and end bad. Dota 2 and other MOBAs reward players that are doing well with experience points and gold, allowing them to out-level and out-gear their competition. In addition to playing better, the winning team becomes objectively stronger than the losers. The odds are stacked in their favor. This is especially true in Dota 2, where players lose money from dying.
Yes, a group of skilled players can overcome these long odds by working together and not losing their cool. However, you can't count on having teammates like that. Some players don't have the knowledge or talent to turn a game around. Others will quit as soon as the game sours. Even the players that do stay around until the end might decide to just berate their teammates or feed kills to the enemies. Then there are the people who genuinely disconnect or remember they've got class in 20 minutes. With teammates like this, you're not going to mount some kind of miraculous comeback.
In short, some matches are hopeless. Dota 2, however, makes you stay until the end out of principle. The lack of a surrender option means you have to endure an extra 20 minutes or so of the enemy team smashing through your defenses, swatting away your counterattacks and eventually leveling your base. If you're unlucky, your opponents will be the sort who hang out in your base and wait for you to come out of your spawn point so they can score some easy kills.
There's no one in this situation that wouldn't benefit from a surrender option. The losing team would be able to concede and save themselves some aggravation. The winners still get the satisfaction of a win without the tedium of mopping up opponents that can't muster competition. Everyone saves time.
Comebacks are a satisfying part of any MOBA but they still exist in games that let you surrender. Will players potentially bow out of games they could've turned around? Sure. But they'll definitely escape games that they had no chance of winning. The reason that comebacks are such satisfying games is that they're balanced. Each team has a shot at a win and knows that the match could hinge on the very next play. A surrender option lets players leave the hopeless matches behind and potentially find one of those legendary bouts decided in the last five minutes. What's so bad about that?