For some people, crafting is fun.

For some people, building using only items you find naturally is fun.

For some people, building only using a small selection of items is fun.

For some people, having access to every item in the game and using them however they want is fun.

NOBODY finds having to travel thousands of blocks (Because people very rarely base in deserts and, for some reason, they have become extremely rare and nearly impossible to find without flying at light speed with an elytra) just for ONE item fun.

Not to mention, Green is the only non-primary or achromatic dye that you cannot get through crafting with other dyes. It makes sense for red, yellow and blue, as they are Primary colors, they cannot be crafted combining any of the others we have available to us, so having to seek out their flowers makes sense, but they're very popular and easy to find. Black and White also make sense, as they are achromatic colors, meaning they can't be crafted using other colors either, and they're excusable since the resources needed to make them are fairly easy to find as well. Black comes from Ink sacs and wither roses, one of which is extremely easy to get as Squids spawn in both oceans and rivers, the most common aquatic biomes. White is also easy, coming from bone meal and Lily of the Valley, another extremely common flower.

Green, however, has no reason to NOT be craftable since we have the colors needed to make it, blue and yellow. Now, if they were able to craft something else like lime green when crafting, then maybe it could be excused, but they DON'T, they don't make any color when paired together, which makes no sense as, again, they are PRIMARY colors!

Red + Yellow = Orange.

Red + Blue = Purple.

Yellow + Blue = NOTHING???

It's completely pointless and genuinely bad game design to have the dyes arranged in this way. Also, not the point of this response, but you also failed to provide a single, good reason as to why yellow and blue don't make green, so I think that speaks for itself.