As so often, the answer is a combination of local tradition and economic reality.

Most UK people who travel to the cottages are going for the whole week, the places which rent them out are geared to that and do not want to rent them out for shorter periods as that will result in the cottages standing empty, unpaid, for times that are too short for others to rent them.

I see something like that in the Netherlands, where the typical holiday park cottage is available for weeks, weekends and midweek periods, with the weekends going at a premium price, and the midweek periods going cheaper as those are used as fillers.
The 'week' periods often change on the Fridays, same as the start of the weekends.

In the UK there is less of a tradition of 'weekend' and 'midweek' rentals, so holiday providers prefer people to rent for the whole of the week.

Hotels and B&B people are more used to people comming for just one or two days, and the owners know that if they have their accommodation empty for a few days, they might get people to come in on the day itself.
They often still offer lower rates for weeks or ask for minimum periods, to get the most out of the market.

But as with all kinds of accommodation everywhere, ask.

There will be cottages that go empty for weeks on end and the owners might be happy to rent it to you instead.  
You may need to go through a search site, go airbnb or call many parks, but I would be surprised if nobody will rent out self-cartered accommodation for less than a week.