Why do computers have firewalls and mobile devices (phones, tablets) do not?

I've yet to find a good answer on this topic. I'm not a cybersecurity expert by any means, but have recently studied the importance of firewalls and am still unsure why mobile devices do not have an actual firewall. On computers with Windows and Mac OS, there is a built in firewall. However, more people are using mobile devices nowadays over computers, so it doesn't completely make sense why firewalls aren't discussed when it comes to phones and tablets. Right …

computers cybersecurity devices firewalls mobile mobile devices phones