It's usually even more expensive to support receiving messages than sending them, beyond keywords like Unsubscribe. If you want any sort of threading its going to be extra. Also its extra for dedicated shortcodes. When you get an SMS from a random shortcode, there might be multiple companies using that code, but they mix the pools enough that its unlikely you will receive two messages from two companies from the same code. Also shortcodes are usually country/region locked. So if you want to international support, you need to buy shortcodes in multiple regions, and different regions have different telco laws. On top of that, provisioning is very manual compared to the modern cloud.

I supported a marketing platform for a while, and it was so much easier to send an email than an sms.