I think there's also a generational/culture shift element to it in two ways (and I say this as a younger-ish adult):
1) Unlike other services, like delivery or driving, which are commonplace and therefore people know you need to tip, baggage handling is not something that a person experiences/deals with on a regular basis anymore. Think about it - if you're checking into most hotels in the US that aren't part of the Disney experience, how often does someone help with bags? It's not something people are used to thinking about and remembering to tip for.
2) Also, I can't remember the last time I carried cash. A few weeks ago, when the topic came up among my friends at dinner, not one of us had a dollar (or twenty, or any denomination of paper money) in our wallets, pockets, or bags. When we take Lyft/Uber/a cab or get delivery or eat out, you can do the tip on card or by electronic means, so unlike my parents (who always have at least some cash in their wallets, usually making sure some of it is in small bills, particularly when they travel), it doesn't occur to a lot of people now to keep small cash on them in case you need to tip someone. Particularly on vacation, and especially to Disney where you plan on using either card or your magic band to pay for everything, it wouldn't be first on a lot of people's minds...until you get on the bus and realize it but can't exactly create an ATM out of thin air, or hold up the driver to ask them to wait while you go pull out money somewhere.