Big chunks of Europe don't get cold enough for the delta to be unacceptable. A 20K delta (so e.g. you want it to be 20° C inside, but the outside air temperature is 0° C) is still well inside the range where the heat pump has > 100% efficiency.
When it's really cold, which even in Northern Europe is only a few days per year, air source heat pumps would be < 100% efficient and you'd step to resistive heating, but it's weird to optimise for a worst case scenario.
And ground source heat pumps are a thing, unlike the air the ground has a relatively stable temperature we can leverage to stabilise home temperatures.
When it's really cold, which even in Northern Europe is only a few days per year, air source heat pumps would be < 100% efficient and you'd step to resistive heating, but it's weird to optimise for a worst case scenario.
And ground source heat pumps are a thing, unlike the air the ground has a relatively stable temperature we can leverage to stabilise home temperatures.