Increasing service is very difficult to do. For example, before the pandemic I would take the red line in LA which would be packed to the brim by the time it rolled into downtown LA, not enough room to even turn around while standing. During my commuting I would do a lot of reading about transit and about the redline in particular.
To increase the capacity of the red line would take a lot of work that would not be cheap. For instance, the length of the trains could be increased, but to do that you have to construct new stations, since the train is already the length of the entire platform, at least the ones used for rush hour. LA has actually lengthened platforms that are on the surface before to accomodate longer light rail trains, but underground this is so much more difficult.
You could lower headways from the 10 minutes they are currently at, but this becomes a physics problem fast. One issue is a lack of turnback stations at the ends of the line so the train has to come to a stop then 'reverse' at the end. Another issue is a subway with a train works like a pneumatic tube, there is a volume of air moving that needs sufficient ventilation, which is why you see these big ventilation grates on sidewalks where subways run below, and to run more frequent trains would require significant upgrades to the ventilation systems along the entire line.