Shakespeare's omission of 'as' before 'single' in 'When sorrows come, they come ...

When Shakespeare wrote: "When sorrows come, they come not single spies, but in battalions", why didn't he put an "as" before "single spies"?

In this sort of context, where the nominal is a secondary subject complement, EME did not require as with nominals, just as we still don't require it with adjectivals:

He came galumphing back.
He came home defeated.

Here's a non-Shakespearean example:

Firſt, [God] comes a Little-One : to ſshew, that hee is food for Little-Ones ; [...] A little One hee comes, from the leaſt of the cities ; Little, of Accompt : Little, in Appearance ; that we might not be afraid to come neere him. [...] And laſtly, he comes a Little-One to us, to ſhew, we muſt come, as Little-Ones, to him. —Austin, Devotionis Augustinianae Flamma, 1635

So Shakespeare might include as or omit it; and the meter here calls for omission.