Prices asked for are often much higher than the actual resale value.
Very often the needs of renovating and refurbishment are underestimated.
There is a strong competition concerning the standard, which is necessary, to let properties.
The standards increase.
In mayor cities like Hamburg, Berlin or Munich quite a lot of rental houses belong to great companies, which are influenced or owned by the local government.
Big players on the market are also "Baugenossenschaften". These firms invest generally a lot in their properties, The standard of "social - housing" or subsidized housing increases therefore steadily.
Due to this the standard of private owned houses has to be increased also, if the owner wants to a secure income for the next decades.
Besides this, there are some standards which a tenant might ask for - otherwise he might be able and allowed by law to shorten the rent.
Under certain circumstances the tenant has the right to have costly repairs done on your expenses. This is legal!
These aspects very often are "forgotten" if you see an offer by a real estate agent.

he aspects, mentioned above, should be considered and should be part of the estimation.
Estimations, which are just made by multipliers, are worthless.