Yes, it is. Bots are so easy to farm. Plus they don't understand the concept of teamwork. All the bot's going to do is push a lane while you're still getting clobbered in a 4v5 like you were before. And then they get an extra kill off an easy to kill bot. And your team is just going to be even farther behind than you were without a bot. The only reason the bots are even somewhat of a "challenge" is because they cheat and get extra gold, and thus items.
There's also a massive issue where bots don't understand playstyle. If I'm playing a slightly tankier build on let's say a bruiser because that's what my team needs what is the bot going to do? Is it going to figure out that I'm a tank and play that way, or go through it's bot routine and pretend it's going to be the damaging bruiser of the game? That's 2 massively different playstyles you have to code for.
This is a lot easier in a game with less choices. FPS can replace it with a bot because everyone's just trying to kill each other. There's little enough difference in the guns that you can have them still function. When you get to even the complexity of LoL replacing someone with a bot won't work.