Aside from saving money, they're doing it also to generate sales.
In iPhone 15 the missing button will be the feature. Some years later, say in iPhone 18, the re-introduced button will be the feature.
All that happened before with Apple devices. They took away the form factor of the original iPhone SE without replacement just to bring it back with the iPhone 13 Mini. Also they took away the ports of MacBooks to end up with an USB-C port only - and now - the latest 14" MacBooks reintrudice a wide range of ports as a.. can you guess?