"The Beach" vs "A restaurant"

Solution 1:

Because that is how English speakers talk (and write).

There are things that a city has (if they have them) that are used with the, even if there are more than one and a particular one is not being discussed. This is the case for beach. Compare other things that cities or towns or other populated areas have, sometimes in multiples (more than one):

the bank
the library
the hospital
the pharmacy
the grocery store
the park
the beach

For these common locations of a populated area, we use the, even if there's more than one in that city, town, neighborhood, etc. We are not necessarily specifying which bank, library, hospital, pharmacy, grocery store, park, beach, etc. we mean.

See "Uniqueness, Familiarity, and the Definite Article in English" by Birner and Ward:

[T]here is a restricted class of uses of N[oun]P[hrases] containing the definite article that do not require uniqueness to guarantee felicity...In each of the above cases, the definite NP - the hospital, the bank, and the grocery store, respectively - refers to some non-unique and not necessarily familiar entity, yet the use of the definite is felicitous.

So

Let's go to the beach

generally just means "the physical location: beach" that is near the city or town in which we live, even if there is more than one beach. It can also mean "the beach that we usually go to."

Native speakers can also say

Let's go to a beach

This is just not as idiomatic.

As for restaurant, we would say

Let's go to the restaurant

only if we assumed that the listener(s) knew which restaurant we meant. This would, naturally, also be used--for the same reason--if there was only one restaurant in the vicinity.

But if someone called my house and asked for my roommate and I said

He's at the restaurant.

this usage is the same or similar to the first one I mention in this answer. In this case, by saying the restaurant I am not assuming that the caller can identify which restaurant it is.

As for

Let's go to a restaurant

the speaker is not specifying which restaurant.

Solution 2:

I assume you are asking about the idiomatic use of the definite article 'the' versus the indefinite article 'a'.

"The beach" would imply you knew either what beach, or even if it means any beach, the article in use with the word beach has the force of specifying the general destination. Whereas if you say, let's go to the restaurant, it is presumed you are describing a restaurant known to your listener, or one you share commonly and refer to enough to describe it as "the* restaurant.

The Beach is a general term for some destinations or locales in English, like "The mountains," "the city," "the countryside," etc. It serves both as a general and specific in these contexts.