Signing up for stuff is fun. It's usually new (you don't sign up for something you're already doing), there's some excitement and energy involved with the newness, maybe others are signing up too.
Signing up usually involves some commitment -- signing an agreement, filling out medical forms, some form of initiation. That commitment is something which the signer doesn't really know or understand the depth of (at least deep enough to do it successfully). Which is why many who sign up don't have the discipline to show up.

There are 3 specific reasons people sign up and don't show up.
The signee realizes how hard this work truly is and wants none of it. Usually the prospects of completing or being successful in something look a lot easier when we sign up for it than what they actually are once the work starts. How many times have you heard, If I knew it would be this hard, I wouldn't have begun? We usually don't tell signees how hard it will be because we don't want to scare people off, it may be too much information at once, or we want them riding a wave of positivity. There's a good chance the signee isn't listening anyway, too caught up in their own excitement.
Other people stop showing up, and the signee follows the crowd. Most people are followers: They do what everyone else is doing, believe what others believe, think and speak like the people around them. And they have more experience following than they have leading -- or at least more emotional connection to it -- so when there's a choice between doing the new thing or following the crowd, what do you think wins? Thus, when one person quits, others quit too.
The signee realizes that the prizes don't come with just signing up and isn't willing to offer the requisite effort. We usually sign up for things because we're thinking about the rewards that come with it. But life never gives you something for nothing. The effort that is the only thing between signing up and reward is a tough ask for most people. Our use of social media does a great job of sensationalizing rewards while minimizing or completely ignoring the effort required to get the rewards. Not to mention all the people displaying rewards they don't own or haven't earned.

All 3 of the above reasons could be (somewhat) alleviated if:
The person who introduces the signee would be more up-front about what showing up entails. As soon as possible after a sign-up, that conversation needs to happen. This isn't a picnic. You must earn your respect and rewards. Remember the energy you had when you signed up -- there will be days when you really need it. 
Signees would honor what they committed to. Easy to sign up equals easy to quit. Everything balances out.
There were academic courses dedicated to stick-to-itiveness. That's all showing up is. Be there every day, show up to every meeting, do your job even when you're tired and when you're not being watched over. These disciplines equal results.
The cost of signing up was higher. This weeds out the professional sign-uppers. More money to buy-in. A more rigorous vetting processes. A longer, more challenging tryout process.

If showing up is hard for you, I recommend my Bulletproof Mindset course. 8 weeks to get your mind in the right place to show up every day.