"That's a lot of books."
Translation:Das sind viele Bücher.
diese (explicitly plural) refers to a subset of something you had discussed before, i.e. "these ones".
The "ones" stands for whatever you had been discussing, e.g. "books".
But "These books are a lot of books" or "These ones are a lot of books" doesn't make sense.
In the English sentence, you're not referring back to something you had talked about before, but are introducing something new to the conversation -- pointing at something and saying, "That's a lot of books."
Such new topics in German are introduced in the neuter singular, e.g. with das or dies, regardless of the gender of the new thing(s) or how many there are.