Why are there transmission problems?

Because a tranmission is a machine and any machine will eventually fail.

Over time, heat and friction cause wear on the transmission. Proper use and maintenance can extend the life of a transmission, but the gears will still wear over time with ordinary use. Seals can also be damaged by improper maintenance. Things that may cause unnecessary damage include driving the vehicle when the transmission fluid is low, using the wrong fluid or contaminated fluid, or shifting into gear at high speeds.