Trying to find out who the "investor" is depends on the servicer. Some will tell you on the phone if you ask, where as others will act as if it's the codes to order a nuclear missile launch sequence. Why? The answers vary from servicer to servicer. The main reason, especially now, is because you have everyone scouring through their mortgage documents looking for RESPA and TILA violations in order to rescind their mortgages. The servicers figure if they stand firm, you'll go away because you probably can't afford a lawyer who could subpoena your file or if you try to do it pro se, you'll probably not do it correctly and the judge will throw out your request.
The other reason may be because they don't want you contacting the "investor" for various reasons or there could be issues with the transfering affidavits.