The concept of a boss easier than the rest of the game is almost incomprehensible, on some levels. Since the concept of a boss is supposed to be that it's an enemy that is on some level more difficult than regular foes, it seems to go totally against the purpose they were created for to have them be total pushovers.
And yet plenty of bosses are, for a whole variety of reasons. Generally, this tends to be either because their fight scene looked cooler in a cutscene - depriving you of some much needed punching time - or because they had to cut the initially planned battle due to time constraints.
However, this isn't the explanation behind all of them. Plenty of boss battle are made purposefully anti-climactic just because it can be really, really funny to have your expectations of a boss battle completely subverted. Also, a whole lot of games want to save something for the sequel, and you frankly can't blame them for doing so.
Not every boss can be Dark Souls levels of difficulty - and nor would we want them to be.