You absolutely can, but we'd rather leave this part to the professionals (despite the extra cost) simply because you can't be on guard duty 24/7. Work happens, sleep happens, binge watching Succession happens, going to the dog park with your new puppy happens — it's easy to miss an alert if you aren't staring at your phone all day. It's also on you to decide whether or not to contact law enforcement in case of an emergency if an expert isn't doing it for you, which is a pretty big ask.
In most cases, you'll also get more out of your system's mobile app if you sign up for a monitoring plan. We'll use SimpliSafe as an example again: Its service comes standard with a live video feed of your cameras, a system timeline, and the option to tweak some device settings from the app, but that's it. Additional features like water leak alerts, dangerous temperature detection, and even push notifications are locked behind its monitoring plans, which start at $18 a month. (Most companies' offerings sit somewhere in the $20 to $30 range, for what it's worth.)