This is somewhat non-technical question, but it seems like this forum is still the best place for it.
I'm reading Shleifer's Inefficient Markets, where he points out that
[...] for futures and options, close substitutes are usually available, although arbitrage may still require considerable trading
where substitutes refer to other assets that posses similar enough cash flow to justify arbitrage activity. Is there a reason that futures and options have more substitutes than say, equities or debt, for example?