Partly it is because PX90 is that boot-camp, do-it-'til-you-puke macho bs which is designed to beat you down ... and which works really well if you have the drive to really go all-out all the time. if you are like the 90 percent of active people who don't need to go that far (way beyond health and fitness) then you won't meet the PX90 standard and will feel like you aren't good enough.

Biking gets you outside (always a benefit---you cannot think the same thoughts in the same way in a different environment) and it makes you be aware of the outside world all the time (or you will crash or get hit.) Even if you think the same depressing thoughts, you cannot give them full attention because some parts of your brain are busy keeping you upright and on the road.

And also ... some people just like riding (you might find some of them here.) if you are one of those people ... lucky you.