That's just it - these are no more customizable than real buttons! They come with preprinted symbols for their function, so no cost advantage of customization.
We're talking a few dollars per unit for the various capacitive buttons, if that. Steering wheel controls already connect over CAN bus so it's not like you're saving a great length of wire, for example. They still connect to controllers, etc. Even tiny savings add up, but the cost of non-capacitive buttons is something they could recoup with almost zero effort.
Instead, it's almost certainly a design choice: they're easier to design around and 'feel' futuristic. Or that's what the designers and marketers think, anyhow.
The bigger cost-savings are in the touchscreen infotainment systems, where they get to eliminate the physical cost of a ton of buttons while simplifying manufacturing and parts inventory (no having to deal with a bunch of different trim parts to accommodate buttons for various options, etc.). That's probably the biggest reason why consumer pushback against touchscreens has taken so damn long to barely start having an effect.
