Because Gaelic has two to be verbs – the copula is and the sunstantive verb bi.
Tha – form of the bi verb – cannot be used in the meaning this is something, it does not link two nouns. You don’t use to say what or who something is, you only use it to describe sth/sb (to say what they’re like) or to give their whereabouts. Only adjectives, adverbs, and prepositional phrases may be predicates of the bi verb (the thing that goes after is in English).
And in this is good coffee and tea the predicate is good coffee and tea – a noun phrase. You try to say what this is – and notice that in the Gaelic sentence tha cofaidh agus tì math there is no this anywhere, the cofaidh agus tì is the subject (so in English it must translate to coffee and tea are…).
You must use the copula to classify/define or identify sth (say what or who something or somebody is) but the syntax of copula is a bit more complicated and in some sentences it can be completely omitted. See my answer to "Tha sin dona".