how troops supposed to outflank the enemy without ground vehicles?
I mean, yeah, there's all sorts of talk about jammers, scramblers and shields, but those would only affect transporting in the immediate vicinity of the countermeasures. An infantry force could still call for a starship above the battlefield and get moved from one side of the enemy to another, with sufficient clearance from the countermeasures. If the enemy had vehicles, all the worse for him, because those would be easier targets to the weapons overhead than a dispersed bunch of troopers...
We do hear about "hoppers" capable of moving a platoon in "Nor a Battle to the Strong", so vehicular movement is still an element of the modern battle. But vehicles might not be all that common, in comparison with transporters - and vehicles incapable of flight might be unheard of.
Transporters are a bit unreliable, many planets have natural occurrences that prevent beaming and most advanced planets would have counter measures. Relying on star ships for short range transportation is foolish as well, it may too busy dealing with enemy starship in orbit to provide short term logistical help. Having ground forces always rely on air forces for logistics is bad tactics. Ground forces have to be mobile on their own, otherwise it will take them forever to conquer a planet. The only starship the invasion force had were those old Vulcan ships, having those old ships as the total logistical support for an invasion force is not bright. For pure logistic purposes, you want troops to be mobile on their own.