Hold your horses there a second...
You're wrong in stating that you can't copy data from ANY web page and paste it into Numbers. I tested this by creating a mini HTML document with a <TABLE> in it, and it copied exactly as I expected. You can try it yourself. Here's the HTML document I created - just save it as a .html file on your system and open in in Safari and you'll see:
<!DOCTYPE html>
<html>
<head></head>
<body>
<table id="mytable">
<tbody>
<tr>
<th>Letters 1</th>
<td>a</td>
<td>b</td>
<td>c</td>
<td>d</td>
<td>e</td>
<td>f</td>
</tr>
<tr>
<th>Letters 2</th>
<td>g</td>
<td>h</td>
<td>i</td>
<td>j</td>
<td>k</td>
<td>l</td>
</tr>
<tr>
<th>Letters 3</th>
<td>m</td>
<td>m</td>
<td>o</td>
<td>p</td>
<td>q</td>
<td>r</td>
</tr>
<tr>
<th>Letters 4</th>
<td>s</td>
<td>t</td>
<td>u</td>
<td>v</td>
<td>w</td>
<td>v</td>
</tr>
</tbody>
</table>
</body>
</html>
It's not fancy, but it is a valid HTML document/table, and if you copy it, it pastes into Numbers as you'd expect.

The problem comes with complex HTML documents that use various techniques to achieve their layout. They may be using <DIV> and <SPAN> tags rather than <TABLE>, stylesheets for formatting, etc.
I see what you're saying when it comes to these more complex layouts, where Numbers' best-guess as to layout doesn't meet your expectations, but it's not an HTML-aware application, so it's not designed to do that.
As a test, I took some table-like data from Yahoo! Finance's page on the S&P 500:

And pasted it into Numbers:

it seems to have parsed out the individual field labels and values into separate cells and identified (via left/right alignment) text values vs. numbers. Not perfect, but manageable.
For contrast, I tried Excel:

That's arguably worse, where it's melded the field label with its value and you have to jump through extra hoops to extract the pertinent information.
Either way, my point is that spreadsheets (both Numbers and Excel) are not HTML-based applications and there are bound to be some differences, and a lot of the details/success depends on how the source data is compiled and formatted.
Ultimately, if you're trying to get tabulated data from a web site, look for some kind of export option where the site provides the data in CSV or some more spreadsheet-friendly format.