Why does Shakespeare spend so much time on the witches reciting incantations/spells in act 4, scene 1?

The witches' language in this scene is both elaborate and memorable. Shakespeare evidently wants to capture the attention of his audience, and this is particularly pertinent to the time in which the play was written. The witches, to a modern audience, may not seem particularly frightening, but those who watched the play early in the reign of James I and VI would have had genuine concerns about the power of witches and their connection to the devil.
Shakespeare plays upon this here—note the use of numbers in the witches' spell, particularly at the beginning, when they describe how the cat has mew'd "thrice," and the "hedgepig" has "whined" thrice also. This seems to be an allusion to the crowing of the cock three times in the Bible, which prefaces Peter's betrayal of Jesus. This, in combination with the fact that there are three witches, positions the witches in opposition to goodness as represented by Christianity and the Holy Trinity.
Meanwhile, as the spell goes on, the depths of depravity to which the witches will sink becomes more and more obvious. Their ingredients are horrifying, from "poison'd entrails" to "nose of Turk and Tartar's lips" and even the "finger of birth-strangled babe." The witches appear to be preying on people from all echelons of society in order to concoct their powerful charm, something which also would have captured the attention of Shakespeare's audience. Did witches walk among them? And were they truly as frightening as these?
Essentially, this scene is a crowd-pleaser. People enjoy being frightened, to a certain extent (hence the continued popularity of horror films, which often tap into current concerns). While there have certainly been modern productions of Macbeth in which the witches were quite terrifying (an excellent example being Trevor Nunn's production), these are typically very much focused on the concerns of the contemporary audience.