ST_Union behaviour

Does someone know why ST_Union behaves differently with linestrings and
polygons?

If a table contains linestrings that overlap at some points then ST_Union
creates a collection of linestrings  that are splitted at intersections.

If a table contains polygons with overlaps then ST_Union creates a
collection of polygons that are still overlapped and are not splitted (the
result contains only the original polygons).

Thanks in advance.

Antonio