Experts recommend exercising at least twice per week, so the limit of 75 PAI points each day encourages you to be active at least twice per week. We also don't want to encourage people to push themselves too hard in a single day. While some athletes may do that without risking injury, it's not advisable for the rest of us.
Remember your PAI score is calculated over 7 days, so you can earn 100 PAI at your own pace - some people workout hard twice a week, while others earn little by little each day with activities like hiking or yard work.