Why vision cannot count things instantly? If vision can instantly "see" 30x30 grid, why does the brain often need to count rows and columns for understand there is 30 rows and 30 columns? Why does the brain fail to instantly count?
You probably mean instant quantity recognition

Counting requires a rather complex high-level process. For an animal to count you'd need:


Memory
Consciousness (to access and manipulate that memory)
Basic algebra skills
Some basic semantic abstraction mechanism (ie, language) which will probably have to be recursive as well.
Some symbolic mechanism


Honestly, even capable adult struggle counting in binary although on paper it should be far easier than counting in the decimal system.

Anyhow, counting will always be effortful and slow as it is a conscious process (ie, system 2, for which you can perform very roughly 5 calculation per second).

I guess what your really mean is instant quantity recognition.

Useless for survival, useless in modern culture

One simple answer would be that such ability will add little to none to your survival chances. Furthermore, within modern cultural, such ability would give little advantage.

The brain is capable of many great things, for instance, the ability to comprehend this text is nothing short of a 'miracle' - despite decades of research, no computer is capable of such feat. But enter mirror reading (where the text is mirrored on the horizontal plane) and now the brain enters a slow, effortful manual mode. With enough practice, mirror reading should become an automatic process, but what benefit such skill give?

Most animals can't count at all

All animals seem to have survived having no or extremely limited counting ability. For us humans, counting is a fairly recent skill (I'm pretty sure it is dated back to the beginning of the trade era) - us humans have survived without it for most of our past. Visits to uncivilised tribes revealed that some can only count to around 3 (then it's "more than 3" or "many") - odd to us as it may sound to us, they are baffled by why would anyone need to count more than 3 things.

One of the fascinating findings about counting is that it seems to be a completely cultural thing - experiments done with both young children and uncivilised populations suggest an innate exponential, rather than linear, sense of quantity. The explanation for this is that for survival it is much more important to know the ratio than the precise count:


  This tree has twice as many apples than this one.


Is more important than


  This one has 204 apple, this one has 408.




To summarise, there's no reason for our primal brain to be able to recognise precise quantities instantly, nor there is modern cultural advantage in having such skill.