Why does it take so long for approved submission

You should be submitting as often as possible. But, not 500! Submit up to 10 a day so that your new submission ranking remains high.

Why don't u guys hire more people then? 

You need to be patient.  Stock receives thousands of submissions per week.  Review times vary from 1-3 months, depending on how many other assets are ahead of yours in the wait queue.  

Why does it take so long for some images out of those 500 to be approved.

By @Vision350771902ejx

Moderation is done by humans, and all depends on the workload in the different queues and the moderators that Adobe assigns to those queues. Standard waiting time is 8 weeks, so you were lucky before with your 5 days.

As @Jill_C mentioned, we don't know what algorithm is used to queue up images for review. And it seems to change every few weeks. When Adobe was backlogged with thousands of AI submissions, it wasn't unusual for me to have assets reviewed on weekends and even over the recent holidays. I suspect moderators were putting in some overtime to clear things out. For the past three weeks, I've only had assets reviewed on weekdays, but nothing on weekends. Whereas I used to have between 12 and 20 images reviewed daily, it is now only about 6 to 8. I've also experienced (as have others) having assets reviewed within 24 after uploading while much older assets are still awaiting review. But even that hasn't happened recently and my older assets are being reviewed first.

Point is, the review process keep changing. Hold tight, maintain your 501 limit, and you'll see a breakthrough eventually.

Submitting 500 at once is not recommended; though none of us knows the algorithms that decide which images get reviewed and when, the belief is that submitting small, frequent batches is better for you and better for the Adobe Moderation team. In addition, it's hard to imagine that such a huge batch has allowed you the time to carefully inspect, edit, title and keyword each and every image; thus, it's likely that you'll experience a higher than desirable reject rate. You could be waiting a long time for these to get reviewed. Some Contributors report waits in excess of 2 months.