Wish that was the case in general, but it isn't. The implementation details matter, because:

- There may be other tools better at specific aspects of the product, which you may want to include in the process;

- Special case of the above, the design tool may not carry you 100% of the way, and you don't want the tool to become an anchor at that point;

- There may be other people working on the product, using different tools;

- There is lots of ready-made stuff that can be plugged in to your product to enhance or prettify or optimize it - provided it's compatible with common standards at the implementation level.