As much as I love typing, i can't even begin to hit upon all the reasons.
The major one that sticks out is the time delay. You are expecting your realtor to find the listing, view the listing and then report back to you in time for you to make a decision. Then the realtor is drawing up a contract and sending it to the other realtor for review. That all happens in 24 hours or less? I doubt it. If it takes more than 24 hours, get used to hearing "highest and best please" cause any good deal in this market is gone or with multiple contracts after a few days.
How many houses is this realtor really looking at for you? How long does it take for all that to occur? Furthermore, if you are investing from afar you typically have a vast area you are willing to play in, so how many of these realtors you using? You training them or expecting them all to know everything you want?
I Flip in GA and FL, I'm a licensed agent in FL. I can tell you right now that having direct MLS access in FL is a HUGE advantage over the work I do in GA, the counties i don't have direct MLS access to in FL I see the drawbacks very clearly. Some of those drawbacks are not seeing the history of a listing, seeing realtor comments about flaws and DOM type information.
Included in that, the MLS searches out comp data far quicker and cleaner than anything else out there. I'm forced to use zillow/property appraiser/clerk of the court data in GA. It's painful to have to verify if a comp is really legit and wasn't some owner financed sale or weird family to family transaction. I spend hours chasing down data just to hit a problem that makes me toss all that work out and go to the next asset.
Also, for me anyways I buy and flip what I can. I've done mobile homes in parks, mobile homes on land, SFH, condos, townhouses , duplexes, quads and of course SFH. I can't target a particular asset class if i want to push volume, i simply act like a real estate pawn shop, if its cheap and I can turn a profit with it i'm buying it.
Some of my best deals were assets that I didn't normally want to deal with, but the price forced me to do it. How do you teach some guy that is trying to make 3% that you simply want to make bags of cash, bring whatever he has that looks good. 99.99% will send you the entire MLS.