verb agreement - Why does "there's" work as a contraction for plural items?

While writing recently, I came across a situation where a character said:

There's a lot of chandeliers in here.

When editing, I realized that I wanted to have the sentence sound more formal, and chose to remove the contraction to do so, which is when I realized that it would become:

There are a lot of chandeliers here.

This puzzled me, and puzzles me still. There's is presumably a contraction of "there" and "is", which is, of course, grammatically incorrect when describing a plural. But why is this okay in the contraction?

Answer

Over time, there's has become applicable to both singular and plural nouns. The Cambridge Dictionary explains that this shift has primarily occurred in spoken or informal contexts. If the character were more colloquial or if they did not tend to speak in an especially refined way, they may say that. They risk being called "incorrect" by prescriptivists but in my experience it's never been remarked upon as a spoken error.

However, the use of the contraction does not generalize to

*There is a lot of chandeliers in here.

since the expanded version has not come into use in the same way in standard forms of English.

Why not use there're? I've used it quite a bit in my life, but sources (including a question on that point) point to it being a dialect feature that is less common in standard contexts.

In short, you have three options:

There's a lot of chandeliers in here (informal, more likely spoken)

There are a lot of chandeliers in here (formal, spoken or written)

There're a lot of chandeliers in here (dialect-specific or less common in writing)