There are lots of jobs that have a higher start rate, and have bennies that kick in sooner. Here in CA, In-N-Out Burger is a better paying part time job than UPS on day one. Problem with this argument is there is a point that the UPS compensation package blows by most other service type jobs.
PT people not caring about a pension IMO is a mistake. A lot of part time employees may not necessarily see themselves working at UPS long enough to eventually retire, but end up staying that long. I myself had this outlook on the job. Took a preload job while attending college. No way was I staying until retirement. Flash forward 32 years, I'm now retired from UPS. I'd bet if you did a survey of employees that have 20+ years in, the majority would tell you they never expected to work at UPS as long as they have.
To me, UPS was never about what you started out with, but what you ended up with. When I left, I was making $100,000+, had full medical no copays free prescriptions, 9 weeks vacation annually, sick days, union protection, and a pension/finish line. No burger joint, hardware store, or even FedEx have that potential.
All that being said, UPS needs to step up to a decent starting wage.