I bought a rental property with a big mortgage - I don’t make any money on it so why do I pay tax?

You pay tax on the profit you make each year. That’s the rental income less costs and one of those cost is the mortgage - but only the interest.

The capital part of the payments doesn’t count as it doesn’t cost you anything (every £1 of capital you pay means you owe £1 less to the lender so you remain even stevens and if you remain even stevens then it didn’t cost you anything).