There are 7-day season tickets, but why there are no 5-working day season tickets?

This might seem like a simple question, but I have always wondered why we have to buy 7-day season tickets, whilst the majority of commuters only use them to travel between Monday to Friday?

I mean, I don't need to buy those 2 extra days, which only makes tickets unnecessarily more expensive.  

Implementing this wouldn't require changing the gates (given that they read a start date and an end date). It will be just the matter of adding that option in the vending machines and in the website. 

For a start, this type of tickets would only make sense to buy them on Monday or a Tuesday, it will still be cheaper than buying a 7-day season ticket. 

Of course, the real treat would be that the price-per-day should remain equivalent to the 7-day season ticket, that is, if the 7-day ticket is  £30, so the price per day is £4.28, the 5-working day ticket should be 5 x 4.28 = £21.40. Fantastic saving!!