Wouldn't cost much extra to fit wheel wells with either motion detectors or webcams.
When you're dealing with something weighing perhaps more than 300 tonnes that's in motion and rattling away over sometimes rough concrete I'm not sure how useful motion detectors are going to be.
As for cameras, might be a "nice to have" but there's more than enough safety critical stuff that needs to be actioned inside the flightdeck during the taxi-out and line-up without adding a "stowaways...checked" line to the before take - off checklist. That has to be the job of the airport authorities.