Hi @kobi and welcome to the checkmk community.
As far as i know all hosts discovered by the network scan are created disabled by default. Just mark your hosts and mass change them to enabled. (just a few clicks)
The reason why they are created disabled is, you can do the network scan automatically and you don’t like to add hosts they are just shortly plugged in to the network as the scan runs. You always like to review your scanned hosts and may will adjust some values.