Why does Germany have so many UNESCO sites?

[Graham]

There's a housing estate in that German list.

[Dr_Maul]

It takes more time, money, and assets to upkeep a modern structure compared to something like Persepolis.

Correct, but the point is that nobody really cared about the upkeep until recently.
Where buildings made in the Middle Ages were constantly being used, as in daily use normally up until the modern age, nobody even cared about Roman ruins for the past millennia. You can expect that a bunch of them were destroyed or built over in that time period.