Dave,
They believed it because it was true.
If you are now saying that not everyone believed it you have changed your stance and that is fine.

It was true that it made economic sense to exploit the workers.
Everyone knew that, but some believed exploitation was wrong and a few, the reformers, were prepared to make a sacrifice for what they believed. OK?

there was a net gain in profitability in a lot of cases.

Really. I would like to see evidence of that, and even if true they were making a huge investment in housing and facilities that their competitors were saving.

It is now considered sound business practice to look after your workforce.

I think we are moving backwards on that with practises like zero hours, unpaid interns, self employed employees, etc.
I am sure employers would revive all the old exploitations too if they could get away with it.

jim,
Slavery was part of British society until it was abolished in 1807

No it was not. And there were no serfs then either. Any slave bought overseas was free on reaching Britain.