Because there is no logical consistency to the attributes used to define each grouping. They're just a hodgepodge of loosely related (at best) ideas. So a lot of people simply don't fit in either category.
For example: I am absolutely pro-choice on the abortion issue. I am also rabidly pro-gun on the second amendment issue. So do I fall into the "left" bucket or the "right" bucket? The answer is "neither".
To the extent that I care about, or claim any affiliation with any of these terms, I could probably loosely be described as a "classical liberal". I don't use the term much though, because most people don't appreciate the nuanced distinction between that and the modern usage of "liberal" (at least here in the US).
I'm also a little more radical than most people, and if I have to pick a label for myself, I'm more likely to choose "voluntaryist", "anarcho-capitalist", or "libertarian".
Liberal doesn't say whether someone is left-wing or right-wing, so we can disregard it as an idealistic person who refuses to take an opinion of the real political question: left or right?
but can't you define your grouping by the disapproval for other group. eg: You are a red group if disapprove blue group.
Even in your own example you can fall into either group by selecting what you care about the most and giving up others. Group where others in the group have made similar compromises.
Otherwise how can you possibly form a group, there is no one else is the world that has the precisely the same preferences as me.
you can fall into either group by selecting what you care about the most and giving up others. Group where others in the group have made similar compromises.
Sure, you could do that, and people do. The point though, is that it makes these terms mostly useless for actually describing anything. Let's say I chose to define myself as "right" because I care more about the second amendment than abortion. You see me self-identify as "right" and then conclude that I oppose abortion, support tight border control, want a legal system defined by Judeo-Christian ethics, etc. But all of those conclusions would actually be wrong. And the same kind of construction could be applied to what would happen in the other case.
That said, of course there are some people who just happen to fit exactly into the bucket of "right" or "left" as defined by colloquial usage. But I still find that the terms are mostly useless because they lack any kind of logical consistency and because so few people actually have that "exact fit". But, that's just me.
why isn't left/right a valid grouping then?