Because nothing is faster and more responsive than direct hardware logic.
"a simple microcontroller is usually cheaper than a 555 nowadays, often doesn't require external components,"
Often? Every UC I've ever used has required a whole slew of caps and resistors just to get the thing to take in operative firmware through a programming port. Even the simple light flashers for vehicles that I've made using a UC and accelerometer need at least two caps and two resistors to make a proper circuit that allows for flashing info to the controller.
"so even if all you wanted was a single function like an edge-triggered pulse, or generate a single frequency, it probably still makes sense to use a microcontroller from a board design perspective."
Frequency generation? Inductor, capacitor, input voltage. Zero UC required and guaranteed to be cheaper.
"As soon as you want anything slightly more complicated, odds are you can replace a ton of other circuitry on the board with that single chip and a small program."
And accomplish things at a glacial speed that a basic hardware-only solution would've solved. As an example - BOSS pedals have basically zero latency because it is all analog. All these newer Line 6 and POD and other digital FX pedal makers have horrible latency, some I've measured past 50ms (almost as bad as trying to live-monitor a Windows Audio device.) It has been this way for the over 30 years I've been playing guitar.
Most times, raw hardware with zero software is THE way to go. Anything else is just a performance loss.
