Bitcoin core has the ability to prune its blockchain all the way down to 550MB, but monero after pruning is 25GB. Why can't monero also give users the ability to prune to an amount of our choosing?
Technically, it is possible to decrease the size further.
However, there are diminishing returns, pros and cons of futher pruning (currently, a pruned node can still serve some of the blockchain, by design), pros and cons of ease and speed of accessing data (some data is kept duplicated in the database for lookup purposes), etc.
You bring a comparison with Bitcoin, and one important point of difference between the two is that Monero's RingCT outputs are pretty much never known spent, so you can't drop these ever, whereas Bitcoin can.
See also this great response: https://monero.stackexchange.com/a/12290/12417
An important point is that you still want the pruned nodes to have at least some value to the network, and for that they need to keep some blocks at least.