Rates for new installations are much lower. Complicated to work it out, but seem to be around 4p/kwh. Also, the landlord would not benefit from any feed in tariff as the tenant pays the electric bill and would receive any feed in tariff benefit.

I rented a house in the UK which had solar panels but we never received any money from the electricity generated with it(I knew that's the deal going in, but still).

Most of those houses existed when the rate was high

That's not how it works, it's based on the date of the solar installation, not when the house was built.

But why didn't the owner install solar back then?

Uhm.....back when? In the UK out of the 9 houses I rented so far 8 were 80+ years old, and the last one was "only" 40 years old. Installation of solar panels on houses this old also presents some challenges, as most of them will have original roofs quickly approaching a full century. And if you need to replace the roof just to install some solar panels it quickly becomes an unprofitable idea.

Who cares? Statistically most houses in most places don't have solar. No amount of justification or argument will go back in time and retroactively change that historical fact.

There are other investment options for something that has no liquidity and doesn't result in a profit for 8 years or more.

Does the owner pay electricity in the UK? In the US the tenant pays it directly.

Usually the tenant would pay directly for electricity along with other bills, unless they are just renting a room.

Depends on the tenancy agreement, some are bills included, some are not.