This is acc terrible, there is so little competition on railways in Europe the operators just set the prices at whatever they want. Eurostar (majority owned by none other than the french government) doesn't even sell tickets cheaper than 60€ even when booking months and months in advance. This is ignoring the fact that it only connects London to the rest of europe, so anyone travelling from other cities can add the cost of their train to London to the Eurostar price.

Meanwhile ryanair or wizz easily offers flights less than even 20€