Because the heater controllers fail as well and are not as accurate. Then also tend to be a simple bimetallic switch or other simple mechanical switch that is prone to failure. The controllers are electronicaLly controlled and have a higher lifecycle.
The main reason is redundancy. If the heater is set to turn off slightly higher than the setpoint of the controller, the controller cycles to maintain temperature. If the controller were to fail, the heater would prevent over heating. You would be able to notice the tank was a degree of two warmer than normal, as opposed to having a heater fail on and cook the tank.
There are more complex control schemes to vary temperature or add the ability to remotely monitor if one desired.
By the way, how is DIY heater controller an oxymoron?