Why did the Roman army base on infantry?

[Flavivs Aetivs]

Whilst it's fundamental element of the little paper I'm writing...

I am happy to believe that one of the main reasons the vast majority of the Greek & Roman armies, in the 400BC to 200AD period at least, were light on cavalry was also down to the relative scarcity of the availability of suitably sound and trained horses, let alone the additional logistics needed to sustain them.

Symmachus records that something like 5 of his 9 horses died in the journey from Spain to Rome, with a stop in Gaul. Transporting Horses over sea was a big problem for the Romans.