5 bedrooms is a large family house. I would expect a house to be balanced in terms of living space, upstairs space and bathrooms. This is important for its use and also re-sale value.
An older 5 bed might have a downstairs loo and 2 bathrooms upstairs with 1 being Ensuite. A modern or recently updated 5 bedders would be likely to have 2 ensuites and a family bathroom.
Consider a fully occupied 5 bed house - that is probably at minimum, 2 adults and 4 kids, so 6 people. You definitely need a minimum of 2 full bath/shower rooms and a house of this size would be extremely odd without a downstairs loo.
Unless you're talking about a house which has been extended and stretched out and squeezed to force 5 beds into, 5 bedders are pretty large usually and bathrooms are part of the balance. You might find a Victorian terrace which really is a 3-bedder with living accommodation to match this, but 2 extra bedrooms have been squeezed in. SUch a house might lack the downstairs loo and the odd one might only have 1 bathroom, but this would be extremely unbalanced. 5 beds with even 2 bathrooms and no downstairs loo seems unbalanced too.
I agree about lots of small new builds which cram in lots of ensuites and tiny bedrooms. 5 small bedrooms with 4 or 5 bathrooms isn't a good use of space really. Quite likely, 5 bed houses with 2 singles and 1 or 2 of the doubles being small doubles, won't really feel like the large family house 5 beds suggests.
In my mind, the 5 bed house also needs a pretty big garden too. Might not be the case in London, but elsewhere I'd want a 5 bed to be on a decent sized plot. You do see new build 5 bed townhouses in terraces which really look pretty disappointing in my mind.