Warning: Word-wall. The TL;DR summary here is - large firms that can really do what OP wants the contractor to do will not be incentivized to work for an investor doing a lot of jobs at a reduced rate.
The model proposed by the investors looking to trade price for quantity is a valid model... when you're talking about individual contractors or small-scale subcontractors with limited overhead. AKA: The kind of contractors that just won't be capable of growing with you, or servicing 10 jobs at the same time.
A huge part of what @J Scott proposes is that his favored subs will save administrative time and marketing dollars by not having to scout quite so hard to find their next sale... or spend quite so much time chasing down payments...
To a small operation, that could potentially mean very real cost savings. ASSUMING that the small operation has complete faith in the investor to KEEP ON providing a statistically meaningful portion of their yearly gross. Also, assuming that the investor that is providing this work doesn't provide so much work that the contractor has to add infrastructure to keep up... because odds are, the investor is not going to want to pay for the contractor to add infrastructure.
Of course, this is a false dichotomy to begin with - giving a contractor a steady 5 jobs at a time is not the same as giving them 50 jobs at a time. Most small contractors willing to trade dollars for volume just won't be able to handle a big uptick in quantity of work.
HOWEVER... the trouble spot that OP was presenting was that he was having a hard time getting a contractor of adequate size and capability to handle a LOT of work.
The trouble here is that a good builder/developer/renovator of 'real' size and capabilities (Office, real management, infrastructure to handle several ground crews, maybe an architect/designer on board full time, someone to handle material logistics, tools, etc.) is going to see the volume-for-lower-cost proposal as being weak... very, very weak.
Let me go through the value-add bullet points and tell you why a large, capable firm (not a single dude... a firm) isn't going to care, and will be getting back in their truck as soon as an investor tells them "Give me a good rate and I'll give you 10 hours a second"
- The contractor legitimately gets more work
If I am large enough to handle that much work, I have my own means for getting that work that are in place and working for me. More work is... more work. Ultimately, it's unlikely that your $100-200k per year is going to really move my needle.
- The contractor doesn't have to waste time giving me bids that I will reject (when I have a contractor I like, I don't get multiple bids)
Most good firms playing at the level you need are charging investors for bids, and screen carefully enough that we are already not wasting a statistically meaningful amount of time giving investors non-winning bids.
- The contractor doesn't ever have to worry about getting paid (I always carry my checkbook with me)
This is actually kind of insulting. Sure, Joe the Jackleg has to worry about getting paid. The people who are really in business have contracts and scary-looking lawyers to deal with people who don't pay us. This is... rarely an issue, actually.
- The contractor can save money on advertising costs -- I'll refer him to lots of other investors and he'll keep busy without advertising
The firm you want is already spending so damn much money on advertising it would make most investors sea-sick... they are not worrying about 1 customer making or breaking their yearly figures through word of mouth.
- The contractor doesn't have to worry about someone who wants to cut corners (I don't do that)
A good contractor won't work with someone who wants to cut corners. Again, this isn't really a great reason.
- When a project goes smoothly and comes in on budget, I pay bonuses
A really good firm is going to look at this and realize it for the carrot/stick approach, and be reluctant to even consider it in a real and meaningful way. Bonuses are super duper when you're a lone ranger making $30k/yr personally, but meaningless to a large firm.
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So... what is the solution, you ask?
Most of the time, investors (especially flippers) end up becoming money lenders or builders. (or both) ... for precisely this reason. Competent, well-organized, well-run firms are disincentivized to work for investors at reduced rates doing a lot of work, and small contractors are typically unable to self-manage a high volume of work. This means that the investor either has to start self-managing the rehabs, or get away from the contractor headache entirely.