Sr consonant cluster in English

I've noticed that other than the word Sri Lanka, English doesn't seem to have any words with an SR sound. I find it odd because English has so many words with SHR sound you'd think some English word would have SR instead of SHR. I may be wrong but I don't know of any dialects of English that pronounces SHR words as SR either. You'd think think with all the dialects of English you'd think at least one of them would pronounce words like shroud as sroud. Sh and s are so close to eachother it's almost like English will let you mix any consonant with r except s. Is there a linguistic reason for this?