Some airports have limited or no carts so they cannot and will not commit to it. What they can commit/guarantee is a wheelchair at every station.

Now at many hubs and larger airports having a wheelchair request coincides with a cart for various reasons by default anyways so you may luck out that way.

TLDR: Carts are a support and supplemental equipment to wheelchairs at larger airports and United will not commit to offering something they do not have or have very little system wide and offers a wheelchair as their base service guarantee.