It appears that all steering will be is changing the lane that your avatar is in. It would be easy enough for them to simply allow the user to set what keys or controls they want to use to trigger this action. It would be as simple as going into the settings, having a controls/buttons/keys page, click on ‘turn right’ and press the button or key you want. It could even work if you plugged a game controller in and wanted to use that. This is a problem that has been solved by the gaming developers decades ago.
As for why, there a number of good reasons. First is you should not lock features behind expensive third party accessories. Second is said accessory will not fit everyone’s setups, it doesn’t work with the full trainer bikes or with the kickr climb for example. Third is changing lanes will be incredibly important in races for drafting and breaking away, and something so important should not be held back from your paying customers.