In theory, you could use an NPN transistor in series with a PNP transistor to switch AC. In practice, there are a few issues to consider:
If you are talking about switching AC mains, you would need high voltage transistors
You would need to provide base current of around 1/20 of the collector current you want to switch. Where would you get the power from? With a triac, you only need to provide gate current at the start of a half-cycle, and with (say) 10mA gate current you can switch several amps. So it is a lot easier to provide gate current for a triac than it is to provide base current for BJTs in this situation.
I guess in theory you could provide the base current via a transformer and rectifier. Many years ago (before we had opto triacs), the standard way of firing a triac from isolated electronic circuitry was to send short gate pulses through a pulse transformer.