Even if they could somehow magically install gigawatts of solar power, all those furnaces in people's homes still run on natural gas.
It's just that... Theres so many households and only so few manufacturing/importing/installing capabilities. And the subsidies aren't limitless too :-)
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.