[lm28469]

I mean, people collect stamps, coins, and many other things that seem completely ridiculous and useless to me, why not collect pictures of planes.

Chances are most of your hobbies are as ridiculous as that for most people out of the hobby

[Blikkentrekker]

Almost no one starts a stamp collection in this day and age.

It used to be done simply because there weren't as many forms of superior entertainment as there are now.

[kube-system]

There are plenty of new hobbies that nobody would call “superior forms of entertainment”. (Although it seems strange to gatekeep entertainment when its only utility is to the person enjoying it)

[Symbiote]

No one starts collecting stamps nowadays because, relatively, no-one receives stamps any more. An unusual stamp used to signify receiving an interesting letter -- perhaps a parcel, or a letter from a relative living abroad.

Without that experience, why would anyone start collecting them?

People still collect similar types of useless stuff (bottle caps, beer mats, guitar picks, fountain pens, license plates).