The bottom line is that you need to install Visual Studio 2019 to access the v142 tools.
Along with different versions of Visual Studio (VS2015, VS2017, VS2019), Microsoft also releases different build tool versions as they continue to improve the compiler and provide additional capabilities and to meet updated language standards (C++, C++11, C++17, etc.). See Visual Studio 2015 not detecting v141 (2017) Build tools
See as well this Microsoft blog posting about build tools for VS2017 and accessing the older v140 from VS2015, Visual Studio Build Tools now include the VS2017 and VS2015 MSVC Toolsets.
Many of you have told us that you still need the MSVC v140 toolset
from Visual Studio 2015 to continue building older codebases. We’ve
updated the Visual Studio Build Tools to include the v140 toolset from
Visual Studio 2015 Update 3 including the most recent servicing
release. You might notice that the compiler toolset build version may
not match the version in a full VS 2015 install, even though they are
the same compilers. That happens because we build the full Visual
Studio and the Visual Studio Build Tools in separate branches that may
be built on different days.
The Visual C++ build tools workload in the Visual Studio Build Tools
will install the latest v141 toolset from VS2017 by default. The v140
toolset from VS2015 will install side-by-side with the v141 toolset.
To install them just select the “VC++ 2015.3 v140 toolset for desktop
(x86,x64)” at the bottom of the “Optional” section.
In your case, V142 build tools was released with VS2019. It does not appear that v142 is available for VS2017. The most recent build tools for VS2017 looks to be v141.
You can use the Visual Studio installer to modify the build tools available by adding or removing from the list. This SO posting describes a similar problem but with v140 missing in a VS2015 Visual Studio installation. MSbuild Error: The builds tools for v140 (Platform Toolset = 'v140') cannot be found
However if Microsoft has not released a particular Build Tools version for the Visual Studio you are using then it will not show in the list of available toolsets.
It looks like beginning with VS2015, Microsoft is providing a standard Visual Studio engine that is shared among VS 2015, VS2017, and VS2019 with build tools and various components being updated to provide new features and functionality and new language standards compliance. There are dependencies between the Visual Studio version and what build tools and components can be used with the Visual Studio version, e.g. v142 is not available for VS2017, most probably to provide an incentive for purchasing the newer product.
On installing VS2017 after installing VS2019
As a side note, while doing a bit of discovery on this question, I have found some articles that indicate that if you have a recent Visual Studio installed and then install an earlier version of Visual Studio, the default target build tools can also change to the build tools for the earlier install.
In other words, if you have VS2019 with a default target of v142 already installed and then install VS2017, the default target will change to v141 requiring you to actually set the build target to v142 when using VS2019.