That's the default that's been around probably since Numbers was created.
If you were using Windows, would you expect it to open a spreadsheet in Numbers? Of course not. That's why Windows sets Excel as the app to open a spreadsheet and exactly the same reason Apple sets it to Numbers.
I would assume that in Windows, the user can change a "property" to change what app open which file extension. On a Mac, it's a setting in the Get Info window. Select any spreadsheet file, no matter it's extension 'name', and use the "Open with:" setting to change it to anything you want, although an app that can read/write a spreadsheet file would likely be best. Now, if you'd like the OS to open every single file with that extension, just click the "Change All..." button. Done. Easy peasy. No trashing/deleting needed. BTW, you are still going to get requests to update Numbers, sooner or later... or the next OS update will re-install it automatically. But, since you have edited your Get Info settings, Excel will still be used to open a spreadsheet file. If you don't make that change, it is highly likely Numbers will still be in that setting.
