I don't know why those are so pricey.
The shocks are not even single adjustable.
Another reason to install them on the rear is to free up your suspension so it can work better.
By removing the spring from the lower control arm it allows the suspension to react more quickly and more efficiently.
Also springs for them are available in many different spring rates which allows you to tailor your suspension to your chosen Motorsports or street driving activity. Example a 110 to 120 lb rear spring usually works good for drag racing and the heavier 200 lb and up springs work better for street and road course duties.