In the FOSS world, the main incentive for license deproliferation is the ability to reuse code.
If one project wants to use another project’s code, it is bound by the license under which the other project released its code. If the project later wants to incorporate code from another source with an incompatible license (i.e. such that both licenses cannot be satisfied at the same time), it cannot do so.
Of course every license is issued at the sole discretion of the copyright holder, who can then choose to re-license or dual-license their code. This is “only” a matter of negotiation if the copyright holder is a single person or organization, which is why some projects have Contributor License Agreements (CLAs) under which contributors transfer copyright for their contributions to the maintainer.
If a project has no such agreement in place (plenty of projects don’t), its copyright is distributed across its contributors, and the license terms can only be changed if all of them agree—which is usually impractical.
None of this is an issue if all projects involved are under the same license (or at least compatible license): they can be freely combined, and the resulting code stays under the same license (or the sum of all requirements from all licenses involved). Therefore most FOSS projects choose one of the standard licenses to make their life easier.
In the commercial world, things are a bit different: The code is usually works-for-hire, with the copyright holders being companies rather than individuals, and there are hardly any practical barriers to an interested client company negotiating individual license terms with the vendor of a particular piece of software.
Shared copyright usually happens because of one company licensing a piece of software from another and incorporating it into their own. Here the borders tend to be clearer than in the FOSS world (one copyright holder per component), and parties to the contract usually have each other’s contact information.
In short: Because individual negotiation of license terms is easier in the commercial world, there is less incentive to have standardized licenses.