Regardless if the match seems unwinnable, it’s always worth it to at least try. I’ve seen a lot of people give up way to easily once they realize they will have to work harder than expected. Or they give up just because they blame other players for making them “carry”.
I’d be lying if I didn’t exhibit some of this behavior myself. It is really difficult to understand other player’s decisions and Blizzard does everything in their power to conceal the weakest link (no scoreboard, 3rd person spectating during death, private profiles, etc.)
But giving an option to surrender is just asking for it to be abused somehow. Someone will find a way to use it to their advantage. Also people might surrender early before a disconnected player returns. The matches don’t last long enough to warrant this feature.