You can use a longsword as an improvised bludgeoning weapon.
Essentially, you can use it as a big metal club. This is similar to the way you can choose to do non-lethal damage with a longsword.
If you expressly want to use the hilt to do bludgeoning, your GM may say that it "counts" as improvised brass knuckles (especially if you've got a basket hilt).
Either way, you take a -4.
As to your second question: Because them's the rules.
As the title
Sure, why not? Though of course you only need your GM's opinion, not ours.
And why can't I use my longsword to deal piercing damage?
'cause You'd need to draw it back too far to be feasible in a fight. Longswords are... well, long. Though you could do so when improvising again.
A. Weapon versatility is a feat specifically designed so you can do varying damage types. So it would include half swording (holding the sword by the blade and hitting with the pummel/guard like it was a hammer- yes, this was a real thing, blades are usually not that sharp, just need gloves) as well as piercing.
It was a feat released in the undead hunter's handbook... which means it was specifically designed for "can't I just turn my greatsword sideways and smash these skeletons?"
B. Because the weapon system in D&D (and thus this game through grandfathering) is based upon media rather than any kind of historical realism. So blame errol flynn and the like- is is far, far safer and more visually interesting to do slashes during an extended sword fight; piercing is about efficient, deadly blows which can be harder to draw out and make dramatic when done several times (as opposed to the nice sudden shanking at the end of a fight).