It?s all a part of the ?give and take? of living within the bounds of modern societal norms. Basic human decency dictates that no one of us has a right to regularly intrude upon shared space (in this case aural space) without regard for those we share that space with, but it also follows none of us can reasonably expect the constant and utter absence from that space of those we share it with.
As to why people do it, it?s because humans are social animals, have been for thousands of years. Back then, when we were largely tribal and more closely knit for myriad reasons, gathering for entertainment and food was a constant way of life and everyone within a given tribe participated. There still exists those closely knit tribes (i.e.: friends and family,) but they don?t necessarily all live together, and separate tribes (i.e.: you and your family and friends and me and my family and friends) generally live more closely together (i.e.: a suburban neighborhood is essentially a group of hundreds of tribes) which can lead to conflict. My tribe roasts a pig and dances to drums from 8pm to 8am and your tribe barbecues chicken and sings from 8am to 8pm; 3,000 years ago, we might have been 100 miles apart and rarely our paths would cross, but nowadays we?re less than 100 feet apart and it?s no longer acceptable to wage tribal warfare to express our rancor at being disturbed at inopportune times, so we call the police in lieu of raping each other?s women and burning each other?s homes to the ground.
If these parties are a routine thing, i.e. multiple times a month and this neighbor hasn?t had the courtesy to at least advise in advance or otherwise acknowledge their own lack of courtesy, then yes, you?re well within your right to call the police and report a disturbance, but I?d offer you might try the more diplomatic approach first and try talking to them. If this party is an anomaly, i.e.: once a year or every few months, chalk it up and hope that in the event you yourself ever need to make an excess of noise or otherwise partake of more than your fair share of any of the shared spaces attributed to the other commonly accepted four senses, your neighbor will concede and allow you your moment without fuss or involving the law to the tune of fines and/or public shaming.