Even the early Roman legions were composed of land owning citizens. Partly because they could afford / knew people they could get equipment from. Also likely because you wanted people in the establishment that you could trust armed and the power that went with it.

I thought that land ownership was offered as a reward for service?

I believe the latter was key and democratization of the armed forced was at least one reason the dictators (Julius etc) were able to maintain loyalty of their troops as they eliminated the republic as such. Im not super well read here though.

Higher level, Fukuyamas political order series does a great deep dive into these kinds of topics, really blew my mind, and made many archaic seeming political structures make far more intuitive sense to me afterwards.