Snape's private stores were right inside his office. If Albert simply barged into the office, he would most likely alert Snape. Normally, a Hogwarts professor's bedroom was located in a side chamber right next to their office, so even if Snape were already asleep at this hour, bursting straight in would still be highly unwise.
Albert's original plan had been to create a diversion, having the Weasley twins cause a commotion to lure Snape away, and then he would break into Snape's office to steal the ingredients. However, after taking the Felix Felicis, Albert's very first thought was to go to Greenhouse Three and steal a pot of Mandrake.
Why go to Greenhouse Three to steal a pot of Mandrake?
Chapter 145