2) It is not purely propotional. There is fixed cost on buy-sell transaction. So if 30K TH appreciated to 60K, you are not making 30K but probably 20K. That closing cost trim off is neglectable for good area house but significant on low value houses.
3) More work load to manage bad tenants
4) Harder to find tenants.....
We have to look at overall cashflow not estimated cash flow. A 80K TH rents for 1.3K/month but vacancy for 5 months, I did not see the point