I'm wondering if it's also not heavily dependent on the business structure that you set up for yourself, as a freelancer. It's a one-man company (for the most successful), a self-employed job (for the majority, or so I hear). The former eventually makes "products", while the latter may entertain a permanent rat race, wheel spin.
Difference in point: how much of what you do leads to higher value at fixed cost (notably in time, how long does it take you to deliver), or same value for lower cost (same idea).
I'd wager that, for the one-man company mindset, you tend to think not task but process, automating progressively etc. (need-to-basis optimization, ad hoc 'frameworks' that speed up your particular niche projects, heck just taking work from client A and refactoring it for client B and building an informal "product" eventually from there, that's the story of all freelancers who've been at it for 5+ years as I see it)
How much of the past work (self and tooling improvements) is factored in present value? Can we decorrelate X years of history of a company and estimate its cost/value production simply YOY? I think not. I think the greatest (companies, freelancers) incrementally raise the bar— making the threshold for competition much too high for a newcomer, hence commanding much higher value.
[At the highest end of that spectrum, meet Apple. Even tech giants struggle to meet that bar, whatever goes in it (tech genius? marketing? craftsmanship? whatever. $ at the end of the day)]
Doing the [work hours × $rate × utilization] equation only says so much about what actually goes into your rate (years of being more efficient at "it", those things you do).
Say you begin at $75 in the US after a ramp up of 3-6 months to establish yourself on the market. You survive for 2 years like that, reach ~$100. You realize it's only $50 real, and your newfound expertise is priced higher in the salary job market. So you either raise your rates or go get that money elsewhere. That's why IMHO an "expert freelancer" market only makes sense north of $100-150 in the US (adapt numbers for any country, minus perhaps the "tech not sexy" effect e.g. in Europe where it's not better paid that other intellectual activities).
Right now, you are worth much more than whatever you did this year, and presumably (hopefully for market sanity) it shows in how much you earn.