Well, couple of reasons, actually:
- It's a money thing - since if it's just a 5% (on top of 9% people already pay) it's abut 14% in total. People have other options in this range especially taking into account the increase in cost of living in Dubai. Again, it all comes down to a specific situation and persons goals etc.
- It's a privacy︀ thing - this means that everyone needs to submit the statement showing all your assets︁ and income from them. That's not good as a lot of people are hiding their︂ assets/capital there and if these documents get leaked (as EJARI documents got leaked couple years︃ ago - check "Dubai Unlocked" or "Dubai Leaks") - their assets will get exposed.
- It's a mindset thing - if you've got nothing to report and nothing to pay -︄ it gives you a feeling of freedom and a peace of mind (you don't owe︅ taxes and you can't get fined/jailed if you don't file a report in time or︆ if the report has some mistakes etc). You can just sleep better knowing it's one︇ thing less to worry about. Also paying tax to absolute monarchy as Martin pointed out︈ makes little sense.