Many companies which had a need for customer.io previously a) hired out very expensive people to build it to them (I have a couple of five figure invoices attributed directly to it not existing as an off-the-rack option last year) and b) can trivially go through several hundred bucks just having a single meeting on what to say in the emails.

You can, of course, code it for yourself out of duct tape and bailing wire, and that will pretty much work if you never need the non-technical marketing team to touch it.