The private companies have pushed for a very large increase in the number of operating trains, with around a 50% increase in the past couple of decades, they are trying to squeeze every drop possible out if the tracks.
The train companies are also often making a loss on these routes, commonly subsidised by foreign Europeans governments looking to expand their rail business, Dutch or Italian tax payers should be annoyed at how their taxes are being wasted.
Profits are also capped so these are not potential cash cows people suspect. British tickets are expensive because the government doesn't subsidise the ticket price, which is a separate debate.
The Public Vs private debate will continue until the end of time, the truth is there are benefits to both systems.
Disclaimer, my wife is senior in a rail franchise, as well as being left leaning, fwiw.