With a full-on rewrite, it's mostly because said top engineers have realized that their application has grown from tiny humble beginning to a behemoth with stuff bolted on everywhere, remnants of no-longer-used components never been removed and the likes, while their superiors won't ever grant budget for actual software maintenance (e.g. remove old cruft, update that Symfony release or in some extreme case, actually use a framework instead of hand-written router and ORM code) but they
would be willing to spend money on a complete rewrite.
What these "top engineers" need is a decent CTO, not people like you who come in and tell them to continue working on a tech stack that they have despised to work with for many months now.
Incompetent upper management will only lead to resignations out of frustration, not to progress.