Why do building contractors turn down jobs, such as when the homeowner wishes to provide materials

Contractor here. My opinion only.

Contractors turn down jobs for a million reasons. Maybe they don't agree that your job is 'stable and high paying'. (Parenthetically, you can't know anything about their cost of doing business, so 'high paying' is an exceptionally variable metric.) Maybe they're busy. Maybe you ring alarm bells.

Speaking of alarm bells, the client buying everything sets off alarm bells for some contractors. I'm not saying any of the following applies to you, but here are some beliefs that some contractors hold:

• materials are purchased by the client because the client is cheap. Or they want too big a project. Or they think they're being cheated by the contractor.
• clients buy crappy materials at a big box store that are inconsistent and cause nothing but trouble during install. Install trouble costs time, which might or might not be paid for.
• when the materials are a failure, the contractor shoulders the blame. (Even when it's clearly outlined otherwise in the contract.) The majority of contractors are in business to provide good service, have happy repeat customers, and get paid. Getting blamed for crappy materials gets in the way of all of that.
• smart contractors buy materials to be on site at the right time. Neither too late (which causes schedule havoc) nor too early (which can cause materials to be in the way and/or get damaged). Some clients aren't responsive to those needs.
• when too little of something has been purchased, the contractor has to urgently find more (sometimes paid, sometimes not paid), which again causes schedule havoc.
• when the absolute wrong thing has arrived, a client has less leverage with a supplier to make it right, right away. Schedule havoc.
• complications from materials cost money. Either it comes out of the contractor's pocket (bad for the contractor), or they have fraught conversations with the client where they ask for more money because 'x took double the anticipated time to install' (also bad for the contractor). Some contractors will price these complications in at the outset, which makes them seem markedly more expensive than their competing peers (also bad for the contractor).

I suspect that buying your own materials slices off the top tier of good contractors. They are busy and don't need the headache of a client that rings any alarm bells. You might be opening yourself up to the middle tier of slightly more desperate and slightly less experienced contractors that haven't experienced the trouble described above.

Rant over.

This might not apply to you if you've already purchased materials, but perhaps others will consider this. A good contractor will sit you down to talk about a project. They'll understand your budget and desires. If you are looking for something durable, the contractor will suggest quality materials that they've used in the past and are willing to stand behind. They probably have a relationship with the supplier, so hiccups in the supply chain can get fixed relatively effortlessly. If the project outstrips budget, the good contractor will try to change the scope of the project, rather than throw cheap crap at it to bring the price down. Yes, the contractor will mark up the materials -- that covers discussions with the client, ordering, delivery, checking for defects and dealing with any issues. Plus there will be some profit added. (Don't be scandalized! There's profit built into everything a contractor does. Profit is used to invest in the business, to reward risk, to ensure longterm viability, etc. It's a rare construction business that doesn't try to generate profit.)