Epubcheck is written in a way that is inherently incompatibile with Sigil. The only way to use Epubcheck within Sigil is to call Epubcheck as a command line app and parse the text output.
On top of that to include Epubcheck Sigil would have to require the Java runtime because Epubcheck is written in (you guessed it) Java. This would require users to install a runtime that is a few hundred MB in size to use a single feature.
Bundling a large runtime in order to run an external app is not going to happen. Any time for epub 3 validation is better served by working on Flightcrew which is designed to be used as a library and can work in a large number of programming languages, unlike Epubcheck.