Because they are professionals who do work that can be dangerous to them and the general public?
Costco, which is likely the closest analog to Walmart in terms of scale and logistical requirement, pays the same.
Walmart requires a consistent level of output which they hope can be maintained by keeping truck drivers happy with high pay. Other trucking companies can afford to have some drivers leaves and thus pay them less because they have some leniency for delays which Walmart cannot/will not tolerate
Dirty and dangerous work attracts fewer workers.
High-utility work is worth more to the employer so there will be more job offers.
When you see dirty, dangerous, high-utility work that pays poorly, it's because there's a ton of people who are willing to do it.