But timetables aren’t about a single location, they are about a line of route or network. There might be a gap in one direction* at Liverpool St, but there won’t be at one or more of Bethnal Green Junction, Hackney Downs Junction, Bury Street Junction, Clapton Junction, Coppermill Junction, Broxbourne Junction, Stansted South Junction, Stansted Tunnel, etc etc.
* the one direction is key here. Every one of the ‘gaps’ you highlight is filled with a train arriving into an empty platform at Liverpool St West side, in a conflicting move that prevents a new departure. And the platforms are all used to (almost) maximum occupancy. I’ve seen many a timetable drafted by well meaning people that work brilliantly in one direction...
So you see, it really is full. Otherwise the timetable planners who have refined the current WA timetable for the last decade since its inception in a back room in Colchester would have found space to fit more trains in.