Practical grouping of like minded people is probably what comes to peoples head first, but thats actually the weakest argument I believe.
One of the strongest factors in party politics is our voting system. This doesn’t just encourage parties but a 2 party system. Our winner take all first past the post old voting system pushes towards this idea. I wish we used Approval Voting but thats another subject.
And the real big reason is election law. We have election code that outright codifies parties into law. It defines what a party is and how it operates. I determines ballot access and how things operate in voting and the functioning of government. We have a government with parties essentially written into it at this point. So those in power write all the rules on how remain in power.
A Texas example: Texas Constitution and Statutes
Sadly voters get caught up in the personalities and events of the day. Not many question the very system we are operating under. It has massive effects on their decision making and shapes the entire system they make choices within.