When washing my hair why do I need way more shampoo the 1st time then much less in the second wash?

Is there a way to prevent this so I use less shampoo?

I rinse my hair to get it as wet as possible for at least 1 minute. Should I do it for longer?

When washing my hair why do I need way more shampoo the 1st time then much less in the second wash?
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