The TVMs, as I noted, don't retail the full range of tickets[1]… as for staffed stations, well, yes… however, out of the four staffed stations on that route, only two come close to providing a full days service.
Glasgow Central and East Kilbride cover the majority of the day (e.g. 0600-2325 Monday to Friday, variations at the weekend); however, the other two stations, Giffnock and Clarkston are 0640 - 1344 Monday to Saturday only, and, are single staffed during those times, so, are not necessarily retailing all of the time.
It's quite reasonable, therefore, to expect the opportunity to pay at the terminus if no other opportunity was available to purchase the ticket required for your journey.
[1] One particular example, again, is the Strathclyde concessionary tickets - they can only be purchased from a booking office, or, from a ticket seller (e.g. the guard, or, the sellers at the gateline/excess booths); they cannot be purchased on the TVMs, or, in advance. Another is the D50 tickets, where a similar restriction applies.