Actually It isn't, It is just hard to reverse. In a few cases after disasters the bitcoin blockchain has been reversed up to 50 blocks worth of transactions in the early days of bitcoin, and altcoins that use the same technology and code as bitcoin have also had specific transactions reversed by adding special rules to the code to block specific transactions, in cases where a majority of the entire alt was stolen. As well as 51% attacks reversing transactions on smaller coins. Bitcoin is only difficult to reverse, requiring some majority agreement that it should happen, it is not impossible or even difficult at a conceptual level to do so.

But the fact that they are irreversible is a huge part of the value proposition behind bitcoin. If some central entity could reverse, withhold, freeze and otherwise mess with transactions, bitcoin wouldn't really be different from the existing solutions (Paypal, Visa, wire transfers etc). another thing is that bitcoin is probably not very useful to people who don't trust themselves. Bitcoin requires you to educate yourself and take responsibility for your finances but it also empowers you in ways a centralized system never could.