A multitude of reasons.
Putting a bunch of power armoured troops into one vehicle just makes that one vehicle a massive priority target for opfor. That holds true even after your troops dismount since that vehicle is also their primary power source, so taking that out drastically reduces your power armours field endurance.
With the power source limitation in mind, putting your primary power source in vehicles means your troops need to regularly return to said vehicle and sit in the crew bays and recharge, so you need to either put your vehicles closer to the contact line, thereby increasing the risks to said vehicles, or have your power armoured troops spend a significant proportion of their time ferrying back and forth. With mounts, they can do 90% of combat ops mounted, including combat engagements, and only dismount for specific tasks like clearing small buildings. With mounts, you can also bring spares for redundancy, and depending on how much you want to spend on the mounts, those could easily be combat UAGVs in their own rights, just like a much bigger, more advanced robot dog, so your section could easily dismount and basically double its effective combatants.
You can make the mounts wheeled or tracked, but that won’t provide the versatility of legs, especially in complex terrain such as forests and shell pocked roads and bombed out buildings and minefields etc. Even in urban combat legs holds a significant advantage over other forms of locomotion since cities are designed for legs.
I could go on, but I think you get the point.
