I guess ownership disagrees because often they were the ones pushing for big money signings. And he's managed to gainfully keep his job all this time.
Cashman is objectively a very good GM. The Yankees haven't had a losing season since 1992 - a remarkable stat. Cashman inherited great teams with homegrown talent that allowed them to be good for a while into his tenure, but he successfully navigated the organization through a "rebuild" without ever bottoming out and basically always being in contention. Every other big market team has had really bad years or runs during that period, including other well-run organizations with deep pockets like the Yanks.
And it is not just about throwing around FA dollars to the Tanaka's or Cole's of the world. It is shrewd trades for guys like DiDi, Gleybar, Urshella, Voit and Hicks (among others) -- go look at who we dealt for those guys, and all of them were/are very good to all-star level players at various points in time. It is player development and having a farm system that produces good players and allow you to trade for others. And finding the Tauchman's off the scrap heap. All of that matters. You can point to bad FA signings, big and small, and I can point to great ones, big or small (like CC, Tanaka (big) or DJ the first time (small)).
Yanks have flaws. I never loved the Stanton "fit" from the beginning based on the roster construct, although the Yanks probably had to make that deal given what they were asked to give up. The rotation depth and situational-hitting stuff still worries me. But they are a contender again and a great franchise. Most others would love to have Yankee problems.