I know this is a joke, but usually it's caused by old cartridges. If you replace the valve cartridge then it will probably mix better. This is usually from old/bad seals. This is why new valves mix fine, but as the seals age, mixing performance gets worse.
Pulling old cartridges can be a pain in the butt so probably best to call a plumber if you don't like diy adventures.
First of all, the comfy range for humans is small and it is not even. E.g. most people will find around body temperature of 37°C nice. But 5 °C above that is burning hot, while 32°C is more lukewarm, than cold.
Then the cold water temp is probably between 10 °C and 15°C depending how long it stood. Same of the hot water temperature, where the first bit in the pipe is more like 20°C and then the boiler comes pushing 60°C in.
So the temperature range that gets mixed changes drastically in the beginning and small changes can make for a large difference in the "center" of the mixing, which is also where the comfy temperatures are.
This type of control is known in industry as "bang-bang control." Among other factors, it doesn't help that the delay time between twirling the control and feeling the temperature change is often tens of seconds.
I've often wondered if this is something that could be engineered correctly or is it genuinely a difficult problem to solve with multiple variables (incoming water temps, pressure, etc)
In Japan, we usually have a thermostat to set the max water temperature and most bath fixtures have temperatures written on them (with a little push button safety thing to go over 40c). I don't know why it's not common elsewhere.
A thermostatic mixer is the usual solution. Set your desired temperature and the valve dynamically adjusts the hot and cold flows to produce that output regardless of input temperatures and presures.
Works great until it jams at the "instantly vaporize target" setting. Which reminds me, I must call a plumber...
Joke aside, get a thermostatic mixer. It can even save you water and money by instantly getting the right temperature water instead of standing outside naked for minutes juggling with a settings, while the water is on.
Most thermostatic (non-digital) mixers can be calibrated to a single "default" temperature, but you still have the option to increase/decrease.
I have experience mostly with europen models, where the default is usually around 38-40 celsius, but you can still adjust it about +/- 6-8 degrees.
There are some fancy digital showers with multiple user profiles, but I personally would never buy anything electronic in the shower. It never lasts in my experience.