Big Sur

Why is it that just because i have a laptop which is old I can't upgrade to Big Sur. I can't afford to spend a thousand dollars just to upgrade to a different software. Apple please let old macs upgrade to newer softwares. Now don't get me wrong Apple is an amazing brand it is by far the best tech brand i know, but this is the one flaw about the MacBook that I'm facing and I'm sure that there are many more Mac users out there who are facing the same problem.