Because a full glass canon that tries to initiate will be insta deleted, sometimes before their gap closer even gets them to their destination. Sometimes it's funny watching a full glass xin/lee dive into a team of 5 thinking they can initiate for their team without a single bonus defensive stat.
League is a very fast paced game, tanks die fast because squishies die even faster. In low elo where reaction to the game isn't as flawless as it should be, I'm sure plenty of people feel worthless playing tanks. As the tank, one can only buy so many seconds. If your team doesn't follow up, those seconds are wasted, and you're probably dead because of it, and maybe the rest of your team.
Aswell, in low elo where focus fire isn't as flawless as it should be, glass canon initiators probably feel alot more rewarding being as you don't always get instadeaded trying to dive their team.
Sometimes, the two go hand in hand, you as a tank dive their team, they don't have focus fire, and you and maybe 2 others 3v5 the whole team. Crazy situations happen when different skill levels are put into the mix.
The game will never be balanced for ALL skill levels, and it really makes sense to balance from the top skill levels than from the bottom. Tanks itemization is decently okay. It's really due to the lack of utility in most tank characters kits that make them unviable. A tank with a single cc/utility nowdays is generally considered pretty bad for teamfights, unless it's a huge aoe one like Wukong or Malphite.