Chariots are not “useless in combat”, but they are very expensive to field and only work in pretty narrow circumstances: wide, open, and relatively solid areas

Chariots are not “useless in combat”, but they are very expensive to field and only work in pretty narrow circumstances: wide, open, and relatively solid areas (aka not bogs). These circumstances were not a thing for Roman legions until they reached spain at least, possibly North Africa and the Middle East. As a result they were not part of Roman war doctrines. On the other hand they were very much part of bronze-age Egyptian and Hittite armies. An other factor to their lack of appeal tho may have been improvements in horse riding technique and gear, as well as training. Riders can also be heavily armed, but have less travel restriction, and you can field a warrior per horse rather than need two horses and a driver per.