Regulation, mate:
https://europa.eu/youreurope/citizens/work/work-abroad/rules-working-road-transport/index_en.htm

You can't work overtime due to safety reason according to an agreement between Regulator (Government) and Labour Union. You can't take a rest according to your needs because you're not carrying cargo of your own.

But to make things more unrealistic, you can search the workshop for mod that allows you take a break every short period.

ProTip: Enable dev console.
Turn off the unrealistic sleep system.
Manually monitor your driving hours and take breaks as legally required.
You can use g_set_time in the console to move time forward 45mins (or use warp speed to move time faster while taking your break).

The sleep system is the biggest let down of the game for those wanting more realism and accurate driving hours.

The downside to doing the above is that you could end up with late deliveries, but if you're happen to use mods you can extend the job delivery defaults to allow for manual sleep monitoring.