Because films are on Amazon very soon after their cinema release date, so the value of the commodity is much higher. Films didn't used to be in Blockbuster until many, many months (or sometimes over a year) after their cinema release, so they didn't have the same excitement factor.
And then there's inflation of course...
And the fact that Blockbuster went bust - so the business model didn't actually work anyway (because no-one wanted the thing they were selling any more). The 'product' was priced at an artificially low level for a long time, to try and drive footfall/keep volume up, and no-one would have paid a lot more for it.
And the basic business premise of supply and demand. People pay the prices on Amazon (or whichever streaming service), the uptake is good, and so the price can inch ever higher.
And all the 'free' films to stream have to be funded (or subsidized) somehow.
So lots of reasons really!