It probably isn't that they are sold out, it's more likely that not all trains are in the timetable/ available for booking! There's a big Europe-wide timetable change on 9 June which is affecting what you see in the schedule for after that date.
As to why you are finding the prices "kind of high", that may have more to do with your expectations than anything else, so no idea how to comment on that. It could be that those trains that are in the schedule are so far only available for booking at the BASE price, but I don't know whether that is what you are seeing.
The prices for the available trains are normal. The supersaver tickets at €19.90, when not sold out (which many are already for some trains) are excellent value for travelling 150 miles in 1h 24.
However Intercity trains, when they become available, will often cost less as they are slower and older than the trains already on sale. Their Supersaver tickets will be €9.90 until sold out.Edited: 15 May 2018, 00:55
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You could also look at Trenitalia's competitor https://www.italotreno.it/en , which has a 12.15 departure which will arrive only five minutes after Trenitalia's 11.26, and still has prices at €9.90 on some July dates.
The shopping cart with a line through means that tickets are not yet on sale, not that the train is sold out.