Several characters including villains just walk into where Barry is in his Flash suit unmasked, or testing his speed in street clothes.
It is hopelessly understaffed and was never originally designed for security.
Star Labs is a huge facility that only ever has 2-3 people permanently around. Even after Cisco "upgrades" security to prevent people from just 'walking in', in that same episode someone just walks in and by passes what I can only assume was a stern 'do not enter' sign.
This suggests two things:
- for all of Cisco's brilliance, he is a terrible installer of security systems.
- It isn't easy for a few people and cameras to cover all the entrances of a large research facility that wasn't necessarily designed for security.