It's absurd and caused by fragmentation of services between Flanders, wallonia and Brussels with no coordination between them.
That's brutal. But I just visited (out of curiosity) www.belgiantrain.be and it says 3286 Euro for an "Unlimited Season Ticket" that "can be combined with other transport networks (STIB, TEC and De Lijn)" (I guess those are the local transport networks).
Stib is the Brussels bus service, de lijn is Flanders, TEC is Wallonia.
On a trip basis, i.e. no fixed costs included, no reduction; public transports are often the most expensive form of transportation (when compared to car or plane for longer distances). However, few people pay the full price, especially not those taking the train regularly, and cars have massive fixed costs. So if you can do everything with public transports and don't have a car, you can save a lot of money. If you still need a car for whatever reason but want to use public transports for some trips, then it can become the worst of both worlds (high fixed costs of the car plus high per trip cost of public transportation).
The car has one big advantage (in term of price), is that it can scale to a few people with basically no extra costs, whereas public transportation is usually linear. If you put four friends in your car and drive a few hundred kilometres to go on holidays abroad, the price of public transports will be ridiculous in comparison.
With "usually" being an important proviso: In Germany, there are weekend tickets for up to 5 people. A student ticket (covering the entire state, included in the tuition fee of around 700 EUR/year) often allows the holder to take someone along after 7pm or on weekends.
In Belgium, it's obscenely expensive. In some others, it's super cheap. It depends how much the country cares about making their public transport accessible. Either way it's very inconsistent across Europe but yes there are countries where public transport is highly subsidized and truly cheap and accessible. Which IMO saves a lot of costs elsewhere, it's the right way to do it.
Sadly when there was a plane option instead - that always tended to be way cheaper (London to Scotland for example).
Personally I'd subsidise it more on less used routes just to push people out of cars more, sadly London commuter routes have kinda hit the wall in capacity ad legacy limitations of the old layout/station platform lengths, rail gauge, bridge heights etc of which much was planned for in an era 100 years ago.