Genuine question: Why isn't the theme system feudal?
Not really. As said, the feudal system imply the mix of administrative, military land-based, and fiscal powers.
Themes had only the two first, and their autonomy in these was limited in the contrary of feudal system where it was de facto, if not de jure, let to the nobles.
Limited by the power of Emperor in itself of course, the non-systematical inheritence, the naming of subordonates by Constantinople in order to hijack too powerful families.
It prevented the great families in charge to have a real local domination, even if they had it partially.
I would say it was kind of close of the vassalic system, if you didn't had the concept of vassality as "I give you that in exchange of your service". It was a function.
On the other hand, it was about giving lands to peasants in order to motive them to defend them, while in the feudal system, it's about making the peasant entering into a clientele or the vassality of their suzerain. The tendency was more about having less free (meaning, without suzerain, even if having a suzerain didn't meant being a serve) peasants.
Again, maybe the closest thing OTL, but it doesn't mean much.