I believe that these 'investment portfolio' products first appeared during the asset inflation of >30 years ago.
People wanted into the rising market, but did not know what to buy and, in the manner of free markets everywhere, solutions arose.
Stamps are a pure asset play, offering no income stream, only the promise of capital gains.
Unlike most other asset plays, stamps are also subject to a great deal of fashion, fancy & whimsy.
(The alternatives are asset plays based on continuing & relatively 'hard' human needs, like land & minerals.)
Lastly, stamps (like all elements of culture)
may have a demographic issue.
Q/ Will there be enough new opera lovers to create a return on your 'investment' in opera ephemera?
Q/ Will there be enough new smokers to create a return on your 'investment' in ashtrays?
For further reading, I suggest:
https://goscf.com/t/38527 ... The Investment Meme Must Die
Cheers,
/s/ ikeyPikey (who edited this to correct hand-keyed html-type tags)