Hi!
The reason of avoiding straight parts of the road are that driving on the straight road is actually more tiring for driver. The driver's concentration and focus are harder to achieve and with today's speeds, accidents are more often.
That's what when designing roads there's a limit in length of straight parts and it is actually preferred to directly connect two curves (with clothoid transition curve or similar).

Since in ancient times there weren't cars or other fast means of transport, there wasn't reason not create straight roads where possible.