All 3 of my employers in the past said this as well (“we don’t draft letters without a verbal accept, as a matter of policy”). I thought it was weird but didn’t let that dissuade me, purely because they were the only offers at each time. Why do companies do this? What benefit does this afford them or what risks does it mitigate for them? I never understood it.
This is done partly to prevent people from using written offers as leverage to either negotiate a higher compensation at their current job or another company they are interviewing with.