Because the source code is really not the important part of that equation. A competitor with a slick marketing strategy will steal your business no matter what.
Meanwhile, if you do release your software as FOSS, then you at the very least get a marketing channel (the FOSS community) that's 1) cheap to acquire (any "this week in FOSS" blog or software repo or whatever will jump on spreading it around specifically because it's FOSS) and 2) is on average savvy enough to know that you're the actual encumbent and your competitor is a fraud. Assuming your product is actually good, they'll be inclined to support it and possibly even buy it.
If this is scary to do with your core product, then at least start with the FOSS dependencies of that core product. See also: Valve being one of the "good guys" in the Linux-on-the-desktop movement; their core product (Steam) is closed-source DRM (the literal antithesis of free software), but their free software contributions (especially around Mesa and Wine/Proton/DXVK) more than offset the "evil" of their core product, and so Linux users (myself included) have no qualms throwing money their way.
Meanwhile, if you do release your software as FOSS, then you at the very least get a marketing channel (the FOSS community) that's 1) cheap to acquire (any "this week in FOSS" blog or software repo or whatever will jump on spreading it around specifically because it's FOSS) and 2) is on average savvy enough to know that you're the actual encumbent and your competitor is a fraud. Assuming your product is actually good, they'll be inclined to support it and possibly even buy it.
If this is scary to do with your core product, then at least start with the FOSS dependencies of that core product. See also: Valve being one of the "good guys" in the Linux-on-the-desktop movement; their core product (Steam) is closed-source DRM (the literal antithesis of free software), but their free software contributions (especially around Mesa and Wine/Proton/DXVK) more than offset the "evil" of their core product, and so Linux users (myself included) have no qualms throwing money their way.