Well forgive me, but I thought that was self explanatory...
Anyway, I'll play along so here are some of the reasons DIY GPS approaches, and any other DIY approaches, are not a good idea:
1 You end up killing yourself and other people
Not necessarily if designed with lateral and vertical spacing from terrain and obstacles, and allowance for turning radii.
2 They provide no terrain clearance
Why can a DIY approach NOT not make appropriate allowances?
3 Even if you clear terrain you have no idea if you will clear other obstacles such as TV and radio masts, power lines etc
. Why would you have no idea? If you map the area beforehand, and fly the DIY procedure in VMC beforehand one could make allowances for obstacles, and monitor the vicinity in what is likely to be your home base.
Indeed, and we all know this but, as stated previously people ARE going to make up their own approaches. This being the case, and provided terrain and obstacle clearances are taken into account, and the vicinity is mapped from time to time to check for new obstacles, what other considerations do legally designed approaches take into account?