There may be big differences in different regions, so just for the record this answer accounts for European Spanish.
Generally, Spanish tends to unfavor two consonants in syllable-final position, so words as transporte or extracto tend to be pronounced as /trasporte/ or /estracto/ in relaxed pronunciation, because of the clusters /ns/ and /ks/ being troublesome in a coda position. Hence why sustrato, from substrato, or traspirar, from transpirar, have already made it into the DRAE.
An x in Spanish is pronounced as a /ks/ with the two consonants falling on different syllables, which doesn't present any problem. However, when another consonant follows the x, and because s + consonant isn't possible in an onset position in Spanish, the k and the s are grouped together, resulting in the first of them being dropped.
I disagree that's not how people speak, or that it sounds vulgar. I've actually heard it quite often when people are just not putting too much thought into their diction because the context doesn't require them to do so. Thus, even though the x pronounced as a /ks/ is predominant in Latin America, in Spain the /s/ pronunciation is the norm when followed by a consonant, as the following excerpt from the DPD points out:
X. 2 c. In syllable-final position and followed by a consonant, it is pronounced as /ks/ or /gs/ in American Spanish: excusa [ekskúsa, egskúsa], extremo [ekstrémo, egstrémo]; but in Spain, even though in educated and emphatic speech the pronunciation is the same as the American one, the most widespread pronunciation is /s/: exponer [esponér], exfoliante [esfoliánte].
