In Japan, people are often out of the house for a long time. Men often work long hours and kids have clubs straight after school, so for stay-at-home mums, the bento was a chance to show their appreciation and love for their family, so that's one reason, they put a lot of time and effort in making a nice, well-balanced bento. Character bentos are usually only for kindergarten-aged kids and one reason to make the food cute is to encourage them to eat a more varied and healthy diet. Nowadays, a lot of mums in Japan are working, so more are relying on pre-packaged food and not putting in so much time, I think.
I think, in Western cultures, some parents are also using bento ideas to encourage their kids to eat more healthily. When I was a kid, I had a cheese sandwich, penguin, packet of crisps, etc. for lunch, but nowadays a lot of schools are trying to encourage more healthy eating. But, I do think a lot of the stuff posted online is more Pinterest showing off and not really practical. But, small things like drawing a picture on a banana or cutting sandwiches into shapes don't take that much time and left overs can be eaten at home.