They get comparatively cheaper mortgages because they get 20% tax relief on mortgage interest payments - they used to get 40% until this year! As a home owner why don't I get that? Rent is charged at the market rates, not the cost of the mortgage - for the last few years rents have been going up and mortgages have been getting cheaper which means £££ for landlords.
The problem is that "selling on" is key to the business of BTL. Consider the following scenario:
1. Buy new build flat for £200k for minimum investment (10% @ £20k)
2. Interest-only mortgage of say £600pcm
3. Rent at market rates of £900pcm
4. Wait 10 years, that's £36k gross profit but then like you say you've got costs, tax to pay - so let's be pessimistic and say you've "only" made £25k. I agree that's not a massive profit over 10 years, but still a nice to have (>10% ROI).
5. Sell the flat for £300k - £180k for mortgageco and £120k for you - that's 60% ROI per year for doing basically sod all.
Granted there's risks e.g. voids, and who knows, maybe one day house prices really will crash and all the BTLers will lose out. I can't help feeling though that in that scenario the poor old tenant still loses their home when the mortgageco repossess the property.