For the same reason that I consider 'A boy and a girl is standing over there' to be incorrect.
There are some 'A and B' expressions that are so closely bound together that they are considered a singular idea (examples below), but, in general, more than one noun , however expressed, = plural.
Fish and chips is a traditional meal.
There is some bread and butter on the table.
***** NOT A TEACHER *****
Verona,
(1) We once had a president who used to say, "I feel your pain."
(2) I think that I understand your question. In fact, not all native
speakers agree on this matter.
(3) I have some information from Dos, Don'ts & Maybes of English
Usage published in 1977 by Mr. Theodore M. Bernstein, a writer for the
world-famous and respected The New York Times.
(4) This sentence is perfect:
On his property were three orange trees, two lemon trees, and one grapefruit [tree].
But look at this sentence:
There ___a man standing there and a dog and two children.
The rule calls for "were," but Mr. Bernstein feels that "was"
is preferable because "many writers and readers feel more comfortable
with a singular verb in such close contact with a singular noun."
He says:
"... there are occasions when one should not be too technical."
(5) He gives this "correct" sentence:
"There were a yacht, a plane, a motor launch and weekends with
Princess Margaret and Lord Snowden." [Those two persons were
very famous back in the "old days" of my youth.] But Mr. Bernstein says
that some people would definitely prefer "was" (especially, I guess, in
speech).
The bottom line: You are studying English. So it is probably a good idea
to follow the rules. But when you read sentences that break the rules,
you should not be astonished.