This question seems confused. For one thing, you never define what you mean by “such languages”. A precise formulation of the concepts is important.
No reasonable linguist would support the idea that, e.g. the resemblance between English “brother” and Latin “frater” (and contemporary French “frère” etc.) is just due to chance. There are too many parallel cases of words with similar meaning in English and Latin that look very similar if you just apply some (relatively) simply formulated sound laws; we call these words "cognates". (If you’re interested in historical linguistics, you need to familiarize yourself with the concepts of sound laws and cognacy if you haven’t already.)
So, we’ve established these resemblances are not due to chance. The simplest remaining explanation is that these similar words originally come from one source. (There are other conceivable explanations for non-chance resemblances, e.g. “contact caused convergence of these words’ forms”, but that’s prima facie less likely and there’s no reason to suppose that is the explanation in general.)
Obviously, how these words came to be in multiple extant languages from one original source is a complicated issue.
Pretty much everyone agrees that languages, in general, seem to change continuously. For example, there is a language called “English” today, that is a continuation of a language that existed 200 years ago that was also called “English” which was slightly different.
Also, it doesn’t seem to be common for languages to just appear out of thin air (the only examples that I know of of this apparently happening are sign languages, such as Nicaraguan Sign Language).
So it seems reasonable to suppose that the spoken languages we see today did not originate de novo any time recently, but developed somehow from past languages, and probably they developed specifically by processes of continuous change.
If English developed from past languages by continuous change, and Latin developed from past languages by continuous change, the simplest way of explaining a body of shared vocabulary is to say that if you go far enough back, the language that was going to develop into English was the same as the language that was going to develop into Latin.
Radically different hypotheses like “mass borrowing of the lexicon from one language to another distinct language” are more complicated and would have to be justified by some kind of evidence of this process occuring. But we don’t have evidence that indicates that English was aways a distinct language from Latin, or that the English words cognate to Latin words were borrowed into English at some point.
Obviously there are disputes about the details, like “tree models” vs. “wave models” but these should not be misunderstood as disputes about the existence of a relationship between languages in the same family; they are just different models of how the languages diverged and diversified over time. Just as how disputes in biology about the mechanisms of evolution don’t affect the consensus that it occured, disputes in linguistics about the mechanisms of language change don’t affect the consensus that Indo-European languages are all related.
Also, linguists are extremely aware that reconstructions, e.g. forms like *bʰréh₂tēr for the common ancestor of "brother" and "frère", don't give a perfect description of an actually existing past language. That is the point bytebuster is making in the comments; nobody maintains that our reconstruction actually existed as a real language at some point in prehistory. The reconstruction, if it is done correctly, represents some information about prehistoric spoken forms that has been preserved in extant languages. It's unfortunate that the "Proto-X" names are often used for both concepts; I believe the hypothesized actual spoken language/dialect group is sometimes distinguished with the name of "Common X" (e.g. "Proto-Indo-European" would be our reconstruction of "Common Indo-European", which is whatever was actually spoken).
I guess another related issue is how variable the common spoken source was e.g. is it more accurate to call Common Indo-European a "language" or a "dialect continuum", but that's just dealing with a matter that is fuzzy even today: the distinction between "language" and "dialect".