I believe this is mainly due to the concept of tipping itself. US (and some other countries) tried to make it a regular thing and you’re “forced” to tip no matter what. That is not tipping, that is a service charge.
The concept of tipping, that works if you’re willing to accept it, is that you give some extra because the service provider went for that extra mile of giving you a service of excellence. Therefore it should never be defined upfront.
In portuguese there is a saying that goes: “Those who pay upfront, will have a bad service”. Not always true, but it’s a risky concept.
Anyway, agree that the tipping itself is mostly a word of mouth thing (i can only imagine how many google queries have been done starting with “How much to tip for….”. You could have implemented a suggestion system, meaning, set your own tipping standard for the industry and at the end of the service provided have a feedback loop for the customer to the movers:
“Please, rate my service?”, typical 1–5 stars, if 4 or 5 have an hint to the user: “Reward your mover with a tip”. In here you’d ‘recommend’ an amount which the user could or not follow up to their discretion.