No, it's pretty simple. The math you presented works ONLY IF THE SPREADS ARE IDENTICAL FOR ALL GAMES AT ALL BOOKS.

Sportsbooks don't want you to shop. Having bettors shop is the bane of their existence.

Shopping for the best numbers is absolutely critical. The math presented above works only if the spreads for all of the games are absolutely identical everywhere, which is highly unlikely. If one of the games at the book at which one proposes to bet a parlay is off by just half a point to the worse of somewhere else, buying that half a point (if it's even allowed) flips the math the other way and makes the parlay the inferior bet. And many places do not allow the buying of the extra half point or point with parlays.

So the question presented above ignores the entire theme of shopping for the best numbers, which is a core element of sports betting.