Interfacing with Transistors
Using discrete transistors has benefits:


They are cheap - basic ones are well below 1c in volume.
They have high voltage ratings. A $1 IC today is often 5V rated, and a 1C transistor can switch 80V
They have high current ratings. A 1c transistor can drive 1/2 an amp. A $1 transistor can drive 50amps. Your micro/opamp/whatever can drive 20mA.
They are really really small in SOT963 packages, and come in ones.
The heat making part of the circuit can be put in away from the rest of the circuit
They can be inherently more robust, since they are quite a big lump of silicon, not a few micrometers in the corner of a chip
They are in stock. If you have been in manufacturing for a while, you will suffer PTLD (Post Traumatic Leadtime Depression), a known IC condition leading to acute hair loss.
They are good enough, and good enough is enough