Because you need a specific set of heroes to counter him, most of which are usually played mid and that's the most contested lane as in who gets to play it and most Dooms are happy to have 10 minute naked midas because of jungling and never buy blink. It's like radiance Naga, the counter is much harder to execute than the move.
Best way to fuck him is to just tell everyone to buy blademail and run into stacked march.
Don't those solutions point to a greater problem with Tinker?
So, to counter Tinker, you need:
Very specific heroes, or he'll fuck you. Ergo, any team that goes up against Tinker MUST strictly choose certain heroes or the game will likely be lost. That's lame. Oh, Tinker didn't pick until 15 seconds after the buzzer started and you didn't get the right heroes? Sucks to be you. I know picking and counterpicking is a big part of this game, but there are a lot of heroes, not being a "hard counter" who can still do OKAY against another specific hero.
Then your second solution, a hero, in this case Tinker, forces the ENTIRE TEAM to buy a specific item, blademail, then magically be somewhere Tinker is about to drop marches. That's great in theory, and yeah, we've seen the videos of it, but it's still not a practical solution.
It just seems like there's a problem. But again, I'm relatively new to this game, so what do I know.