Different markets I suppose. On Phones/Tablets they want you to get hooked, spending small amounts but often. There is much criticism of their overly complicated purchasing system. They did have an offer recently to buy all Williams tables, can't remember the exact price, think is was £28?Just checked and you can buy the packs of 3 tables for £7.19 on Pinball FX3 Steam. Much cheaper than on the Android app. Odd.
Steam is much better value but more aimed at the serious gamer rather than casual gamer.