Bicycles have gears which allow you to trade off muscle energy over time. When you're touring with an extra 10 pounds most people just gear down and ride a little longer day. At the end of the day they eat a little more food too.

When backpacking with an extra 5 pounds you might slow down, or take more breaks, but you're body is directly taking the full brunt of the weight for the entire time. In addition when carrying weight up hill you get nothing back going downhill as you do on a bicycle.

Weight always matters in touring and backpacking but trying to equate the two is a mistake in my opinion.