Except that by the time I get to vote in primaries, the nominees are often already chosen. The real way to have lots of choices is to live in New Hampshire and Iowa.
It is indeed like a runoff election that is executed very very poorly.
The winner take all system for awarding electoral votes has caused us to only have a few "swing states" that actually matter.
Not to mention the fact that the parties now control the debates (rather than a nonpartisan organization like the league of women voters). They get to decide who is allowed to participate in the debates. They often change the criteria in order to specifically exclude/include particular candidates.
If you are a 3rd party or an independent candidate, good luck getting anywhere near a debate run by democrats and republicans.