Why is steel extensively used with concrete?

Steel & Portland cement actually have similar thermal expansion rates. It’s really an awesome serendipitous discovery. Modern life would not be possible. Highrise buildings, highways, bridges, tunnels, piers and dams.

The reason brick fell out of favour was that it had a different rate than Portland cement mortar. Thus the freezing & thawing cycle led to cracks. For centuries bricks were bonded with lime mortar. They actually have similar thermal expansion rates too.

Now the problem is that steel corrodes. Leading to spalling. So older stuctures are modified by connecting the steel to sacrificial anodes like zinc. Or by using fiberglass/resin reinforcing bars in new constructions. So far I haven’t seen any articles about the downsides to this combination.