A big problem is that a lot of the members of the party don’t follow the party platform. In fact, I’d be surprised if most of them knew what it was aside from broad issues. So you may have Democrats that vote fairly similarly to Republicans and Republicans who vote fairly similarly to Democrats. So it’s often hard to pin down the ACTUAL difference between the two - for instance, many Republicans and many Democrats are very protectionist while others are very much free traders.
And the differences between the two change quite often. Democrats used to be the big bastion of segregation, now they are far more liberal on that issue.