You must have heard wrong. When you receive something (e.g. a gift) you would ALWAYS say it IS beautiful, they ARE beautiful, and so on... So always a 'present tense'. As far as I know, there is no colloquial language that would use 'were' in this situation.
The (basic) rule is quite simple: the simple past refers to things in a (definite) past without a link to the present.
Compare 'I was a boy scout in my teens' to 'I have been a boy scout all my life', where 'in my teens' is a definite past time.
'They were' would sound very odd here. I suppose it could be used, to make the response quite hurtful - if a man's given a woman flowers, and she says 'Oh, they were beautiful [when I saw them growing in the garden; now they're just temporarily attractive and bound to wilt]. But are you sure you heard right? 'Oh, they're beautiful' seems a likely response, and if the movie was American the 'they're' would come out as two syllables. [In many Br English speakers the sound would be similar, but RBP would give it a single /eə/ diphthong.]
What I think you heard was, "They're beautiful" (contraction of 'they are'), with a thick American accent stressing the "r" sound, and also as is their way, making the word almost into two syllables, that is, they-urr beautiful