Two reasons I can think of in their favour:
1. Tests how you prioritize & manage your time. It is key skill for MBA students and your post MBA career. You would always be on your toes.
2. When you have less time, you would put your thoughts in a raw form by yourself. When applicants have multiple weeks, they try to get artistic and even seek assistance from others. Exactly why cope-paste is disabled.
2.1. The essays are merely asking what’s special about you and why MBA. Shouldn’t take more than 30 minutes to tell this, right?
2.2. Managers are expected to prepare for the worst. The two-steps-ahead ones would have prepared the resume & essay draft by finding out previous year/round questions. It’s always the same Qs in slightly different words.