I purchased my first drone, a DJI Mini, about a year ago. Since that time I've purchased a new Mini 3 and an used Mavic 2. I also own 2 FIMI X8 UAVs. These inexpensive FIMI drones both have an IP43 rating. The FIMI X8 Pro has the same functionality and features as the Air 2 but it's half the price. Clearly it is not expensive to build a drone with an IP43 rating. So why doesn't DJI do this?

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Have you ever tried your FIMI in the rain?  
I've never owned one so no idea.  

But for the Mavic 2 and the Phantom 4, it would definitely deserve an IP43 rate.  I have flown them in rain and snow on multiple occasions, so far no problems to report.  
Of course I always dry them after this type of flight by storing them in a heated and ventilated room.

If I had to guess, it would be because an IP rating of 43 is just useless on a drone...

IP68 is what you really need, to make it worth it...

https://www.amplex.com.au/blog/ip-ratings-explained

Sure IP68 would be nice but IP43 is very useful. I've flown my X8 Pro in moderate rain multiple times. I've never had any problems at all from doing so.

My DJI Mini 3 flew through some low clouds when suddenly the camera tilted up and left. I got a camera error message and landed the drone. I used a small fan in a dry environment to dry it out. It took a couple of days but the Mini 3 is back to normal. However now I'm terrified to fly it around the lake and marina where we keep our house boat. I've been using my X8 Pro instead.