Unfortunately it isn't always a case of just a cheery moment's encounter with staff - it can take time to get staff's attention, they may be engaged with others, or when the manual gate is opened half a dozen other people take the opportunity to barge through.
All of which have happened to me, multiple times. So far I've not missed a train as a result, but I've had quite a few close calls.
They can also be one of the 'awkward' staff who insist on seeing you put the ticket through the gate and it failing, before trying to let you through manually - wasting everyone's time, especially if there is a big queue behind you.
Or they may be hiding away somewhere and require shouting into the ether in order to get them to make an appearance.
Or they could be like the rather odd setup I encountered at Newbury a few months ago, where the only member of gate staff was apparently also responsible for selling tickets to passengers without them. I was waiting there at least five minutes while they dealt with a queue of passengers, including one girl involved in a long debate as she was clearly lying about why she didn't have a ticket, before I just went through the wide gate when somebody came the other way, else I may well have still been waiting there now...
Not to mention the time at Bristol when, after my ticket didn't work the gate, I was motioned to go through a gate that had just opened, only for it to close on me while going through, and after complaining about that slightly painful occurrence was told I should have read the sign about how to go through the gate with luggage. A sign that was behind me. I pointed out that I didn't have eyes in my back, and got quite a bit of abuse in return. (Why do you have to carry your luggage *behind* you to use these gates, anyway? It doesn't help if you have a large shoulder bag, as I did at the time. It only seems to be an issue at GWR stations...)
All are annoying scenarios. I've no objection to ticket gates as long as they either recognise valid tickets, or staff on them are sensible enough to deal with people who have valid tickets that the gate doesn't accept, in an efficient and timely manner. Often they are, but all too often they are not.