I've been creating charts in Excel 2004 for Mac for years and copying them into Illustrator. They have always previously pasted into Illustrator as vectors illustrations. I've created hundreds of charts in Excel before and copied them to Illustrator to "dress them up" for reports with no problems. I just created a chart in Excel 2004 for Mac and copied it to Illustrator and it came in as a picture. This is the first time I have ever had this happen with an Excel chart.
I've searched various sites and have not found anyone who's run into this issue.
Has your system or software changed? Have you tried printing to PDF and opening the PDF in AI?
I just copied and pasted an Excel chart from Excel 2011 Mac into AICS 5 and 6. Both came in as vector. Although the font is messed up.
Mac OSX 10.7.5
The messed up font is from Excel using the Calibri (Theme Body) as the standard font. You might also check a little closer as to the quality of the vectors. The stuff I've been getting recently has been having lines coming in as filled shapes.
I have never printed to pdf from Excel. I have always just copied the actual chart and pasted it into Illustrator, then worked on it there.
The image below is from one of the charts that originated in Excel and then was "dressed up" in Illustrator. It was a fairly complicated chart but it came into Illustrator with no problems whatsoever. (The Excel chart was fairly basic - specifically to avoid any issues when pasting into Illustrator.) All of the drop shadows and other embellishments were made in Illustrator.
My system has not changed - other than the usual MS Office updates.