Because Heimer is completely immobile and has the defense of a wet paper bag. His turrets keep him alive, literally. If you get a jump on him, he has to rely on a delayed slow/stun to get away from you. If he Ult-Q's, you back the hell off because it's supposed to hit like a truck. Once it's down, KEEP GOING AT HIM. If he can't set up his defenses, he's toast.
Meanwhile Mr. Demaglio can hop away from threats as well as throw his goddamn boomerang at you over and over to slow you. Teemo has Move Quick and post-6 he'll just trick you into running over his shrooms.
Also, Heimer just straight up counters some tops since they can't take out the turrets without getting heavily damaged. Though not all of top lane bends to Heimer, or else you'd see him be pick/ban.
You want help on who can go top against him? Cho and Sion come to mind. The former can sit at tower and turn it into a farm lane, while using ranged harass to knock his turrets down (I believe eating them also gives him stacks, TY Heimy!).
With Sion you level E first and proceed to make any Heimerdinger cry as you PUSH HIS TURRETS AWAY. YES THIS WORKS. It's goddamn hilarious too. If you hit Heim with them, all the better, since he'll take more damage than the turret did. Just rush a chunk of health and MR and soon you'll be pushing the wave towards HIM because he can't sit and supervise his turrets farming.
TL;DR Heim can be annoying as hell at times, but he's not godlike. Exploit his weaknesses.