It is not off by default. I have seen a lot of different hardware in my life (I basically do booting for a living), and I have never encountered a BIOS that had PXE disabled by default.
BIOS has a boot order, and as far as I can recall PXE is always the last option. The order of what comes before varies, e.g. whether or not USB boot is tried first.
In fact, on most systems if they cannot boot, they keep rebooting and ending up in a loop trying PXE boot over and over, and then giving up and rebooting again. But it should be noted that of course, if you get access via PXE you already have full control over the system anyway. The new thing here is that you can use a single "malformed" packet with a payload instead of having to set up a DHCP and tftpboot server to deliver a malicious payload. The fact also that this happens before TPM is enabled makes is extra serious, as there is no way to stop any malicious code executing.
BIOS upgrades can be helpful if they contain a fix, but they tend to be unavailable on consumer hardware fairly quickly after the vendor stops selling and supporting it. Also BIOS upgrades are know to be finicky, and it's very easy to brick your system, which means opening up the box and connecting to the chip with a CH341A EEPROM programmer, something which not everyone is going to be capable of. Some machines have way to recover from a bricked BIOS, but in my experience those don't work most of the time and you end up reprogramming anyway. On many laptops the BIOS is on the back of the mainboard which typically means disassembling the system to get the CPU cooler off.