Amazon only allows users to select 30 items at a time from the dropdown list and add them to the cart. Why not let the users type the number themselves in case they want to buy more than 30? Is there a reason for this number?
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I can't see that amazon would limit the number of items for a reason as easily avoided as having a long drop down - a simple UI change would fix this. I think the reason is probably more rooted in business logic. I can't comment internationally but in the UK most retailers limit the amount of a single item a user can buy. An old client of mine (hotel chain) when having supply issues tried to buy supplies from a supermarket and were told they couldn't buy the quantity they required.
While why is X designed like this? is always somewhat speculative, I think it is probably a pretty simple answer in this case:
The drop down is the most convenient way to select quantity, but it requires a limit to be workable.
Adding more than 30 items is extremely rare. This is not something that the average shopper is going to do. And for items where you might want a large quantity, usually one can find a bulk package on Amazon rather than adding many of the individual item.
Putting a work-around on the product page would probably increase support problems. A free text entry box might lead to people accidentally buying large quantities and then needing to cancel their order, among other problems.
It is, however, possible to change the quantity to more than 30 in the shopping cart, which is probably good enough to handle those rare cases where someone wants to buy a large quantity.
The drop down menu becomes too long if more than 30. For individual consumers, the drop down list eases the purchasing process and prevents them from accidentally entering larger quantities. But bad news for sellers and retailers using the 999 trick to check the seller quantity, though they could still manually input 999 before checking out, I'd rather use chrome extension tools like Amzpecty to do the work for me.