So for instance your model might need access to borrowing. In fact I know people whose entire trading strategy depends on being able to access shares to borrow in large size so that they can squeeze out a few bps of yield on certain situations. The guys who control the borrowing are picky about who they want to lend to.
Or your model might need to be really, really fast. Not everyone can just have access to a microwave link. The guys with the microwave are also picky.
Or your model needs certain kinds of data, which you will need to get certain vendors to sell you.
You might have a need to trade multiple models at once, which are not profitable on their own (uncorrelated, share costs). This is a juggling act done often at the business level.
For almost all trading you need some staff who know how to do things. It's not always the case that you just pay them a salary to do some job. Business decisions there as well.
Or your model might need to be really, really fast. Not everyone can just have access to a microwave link. The guys with the microwave are also picky.
Or your model needs certain kinds of data, which you will need to get certain vendors to sell you.
You might have a need to trade multiple models at once, which are not profitable on their own (uncorrelated, share costs). This is a juggling act done often at the business level.
For almost all trading you need some staff who know how to do things. It's not always the case that you just pay them a salary to do some job. Business decisions there as well.