Скачать или смотреть Why Was most Roman Trade Conducted by Sea?

Short answer: Because before the advent of modern land transport like steam trains ships were really the only viable option for long haul bulk transport.

Transport over land was infinitely more cumbersome and costly as the only options available was animal back, cart or carrying by humans.

This all put a limit on land transport.

It does not mean that there was no land transport only that it fell within two categories:
One would be short distance to the nearest river or sea port or second long distance land transport for goods that had enough value to support it.

Within the last can be the movement of military units, which had high value in terms of securing territory.

Within the Roman trade network low value bulk cargo in the form of daily stable foodstuffs was very common, however, and for this the only real option was sea transport.