The reason they make all the previous passengers exit the platform before letting new customers on is so previous passengers can't hide in the crowd and get back on the ride for free. Only by having a totally clear platform for a moment before letting the new passengers on can you ensure you're not being cheated by sly peeps
In the absence of separate loading and unloading stations, you can do things to improve the speed of coaster loading cycles. I have a giga coaster with 12-car trains. It runs 3 trains and has a loading cycle of less than 30 seconds, yet almost always has every seat full. When 1 train leaves the station, the next pulls in only a couple seconds later. So with some tweaking, you can get some huge throughput on your rides. Here are some of the tricks I use.
1. Single-Car Coasters and Track Rides
Put the exit gate directly in line with the yellow exit arrows painted on the station platform. That way, the old passengers go straight off the platform in the minimal time. Entrance position doesn't matter so much because new passengers sprint to their seats, but in general it should be as close to the exit (and thus the position of the car) as possible. If the entrance and exit are on the same side of the platform, put the entrance adjacent to the exit. If they're on opposite sides, put them directly across from each other.
2. Multi-Car Coasters
Speeding the unloading process is ONLY possible if the entrance and exit gates are on OPPOSITE sides of the platform. If they're on the same side, then the yellow exit arrow path will by at 1 end of the platform, so old passengers at the other end of the train will have to walk the entire length of the platform, which takes a LONG time. OTOH, if the entrance and exits are on opposite sides of the platform, you can put the exit gate even with the central car of the train near the middle of the platform, which minimizes the distance old passengers at the ends of the train have to walk, and thus the time required to start loading.
Also note that as train length increases, you have to put the entrance gate closer to the rear end of the train. This is because the entrance gate closes shortly after the train leaves the station (or maybe just before the next train is about to enter). Anyway, if you're running a tight schedule with very little time between trains, often the entrance closes before all the slots on the platform are full, and the slots fill from front to back. When the next train arrives and the entrance opens, guests at the front of the queue can dash to fill the last few open slots, but only if they don't have to go very far. So putting the entrance at the rear of the train lets you fill every seat (minus the odd empty seat in the last car due to # of remaining seats < size of next peep group in queue).
Being able to do this, however, requires careful design of the coaster up front. You should determine how many trains (of how many cars) you want to run up front, then design the track so you can do it. The length of the track and the average speed of the coaster needs to be such that it can be divided by the number of trains desired with only a small remainder. Then you have to set the max waiting time to be (ride duration) / (number of trains), which should be on the order of 30-35 seconds. Then place the entrance and exit gates as described above and things should go very well.