Say you have 10 people who want to run for the office of President: 5 with an R after their name and 5 with a D after their name (just for simplicity we’ll leave out 3rd parties).
Since neither party enforces adherence to the individual planks in their platform, you end up with someone who is .35R/.27D (where a candidate supports 35% of the republican platform, and 27% of the democrat platform). That might be just the kind of person YOU are, so THAT candidate best represents YOUR views; but, because of primaries, other states who hold their primary elections before your state get to pick who the two candidates are going to be.
In which case, Joe Voter goes in to the polls on final election day and sees only two people, a D and an R… let’s say he’s more R than D, so he picks the guy/gal with the R next to their name (and so does every other voter).
In effect, we’ve given up our choice as individuals to the choice of the parties. The parties have the power, not the people.
We need to return to a party-less system, and back to a government "for the people, of the people, and by the people."