Herd Sires for sale

Hands down the single most important part of your herd's puzzle. While a cow has one calf per year and only affects the genetic of that one calf a herd sire influences the entire calf crop. Why do some folks invest a good amount of money in a single cow but then don't invest reflected resources into a better bull? In a herd of 20 cows, a single cow has influence on 2.5% of that year's calf crop while a bull has 50% influence. Increase that herd size to 40 cows and a single cow decreases influences from 2.5% to 1.25% but the bull still influences 50%. Why not buy the best bull you can?