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 grocery store
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.
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.
Let's go to a restaurant
the speaker is not specifying which restaurant.