Because once you run metal washers the way the strut tower bar handles the load/tension is changed since it's no longer flush. If you're running rubber washers you no longer even have a solid mount. If you are going for just the look of one one I s'pose, however if you're going for actual function (to actually improve rigidity) and correct geometry the correct way is to lower the engine. The strut bar the way Ford intended it to be mounted (triangulation with 3 points tying it into the firewall/strut towers and not just the bar alone) acts as a spring. Using washers or spacers changes the way the load/tension is distributed, using rubber washers just makes it a decoration piece that no longer/barely even acts as a spring/solid mount. If you're going for looks/decorative piece and a strut bar that does not function as the factory ones do, go right ahead with washers. Lastly, if installed correctly the way they were engineered to function, you will not have any NVH, these are installed on factory cars that have to pass strict NVH guidelines. Just putting this information out there for those who do want to improve their suspension.
