That's because ACCESS uses the combination of carriage return and line feed
to mark the end of a text line. EXCEL uses just the line feed character. So
if you want to have multiple lines in the cell from the ACCESS field, you
either need to use a calculated field in the query that you must export
(where the calculated field strips the carriage return character from the
text string), or you need to convert the text string in the EXCEL cell by
stripping it there.
It's easier to do it in the query that is being exported. The calculated
field would look something like this:
MyTextString: Replace([FieldName], Chr(13), "", 1, -1, 1)