Why does a system boiler with hot water cylinder not use a diverter valve?
Radiators don't put out a lot of heat at that temperature, so you'd need a lot more area, so the heating loop usually runs warmer than you'd really want the hot water cylinder to get up to, so the boiler thermostat setting is a bit of a compromise with a system with no diverter.

Combi boilers that are either heat or HW but not both, often do have different temperature settings for the two modes, determined electronically using a thermistor sensor on the boiler, but regular systems with CH/CH+HW/HW modes selected through a timer plus room and HW tank thermostats, usually have a single mechanical boiler thermostat setting, since the boiler's control system is usually pretty simple and independent of the external control circuitry. On old pilot light boilers, there may not be any electrical circuitry around the boiler at all, just a mechanical thermostat operated gas valve.