I could get that for you, but I won't.

For extremely busy restaurants / cafe's where people are already waiting long periods, they probably don't want to overcomplicate things too, and increase the risk . They'd have to keep 2 different milk frothing machines, and every time a customer got sick, risk getting sued, whilst slowing down the efficiency of orders.

Whilst it might increase the number of potential customers, in practice, it might only have negatives