Not being able to buy single items from a stack on the marketward.

[Wazabi]

You think Walmart would dedicate a whole store to selling 40 cartons of 12 eggs for 2.49 each?

No...they'll find a balance by studying the demand pattern...hence you'll see only 1 shelve of eggs rather than the whole store.
Same goes for FF's market...I seldom if ever make more than 4 posting of the same item. Putting up 20 posting for 1pc of the same item is a waste of retainer slot unless the item is really in demand (for 1pc at that price, taking into account of potential undercutting).

I do not think it is "Lazy" people as you say. Some of us can not spend or do not have hours to watch minute market fluctuations and adjust accordingly.

You're right, some of us are working adults with other commitments. I get at most 2 hours on weekdays, and I spend at least 30mins of that session to trade. Some are better at optimizing their time, some are not...but you don't have to watch the market for hours and keep making adjustments in order to trade.

Retainers have 20 spots. Do you really expect a seller to fill those up with 1 item sales?

You're probably not optimizing your profitability if you have to do that....