Actually, "present simple" tense is used not only to describe current habits or routines but also to describe current states or actions that are happening right now. In the example you provided, "this guy buys flowers" and "you want to sleep" are both describing actions or states that are happening in the present moment, which is why they are in the present simple tense. So, even if the actions are not habitual or routine, they can still be described using the present simple tense if they are happening now.

In the first example you would normally use the present continuous if he is doing it now, and the present simple for something he does regularly. In the second example, using the verb ‘want’, we do not normally use the present continuous, even though the ‘action’ is current. Many ‘stative’ verbs in English follow this pattern, and to a British ear, or at least an old one like mine ‘I’m loving it’ [an advert slogan of a well-known fast food enterprise] sounds strange. Love, like, want, dislike, hate, etc. routinely use the present simple in all situations.

‘Buying’ is observable. Therefore: This guy buys flowers. (Habitually. In the past, present and future) This guy is buying flowers.(Now, in the present) ‘Wanting’ isn’t observable. You want to sleep. (Now, in the present) You always want to sleep. (Habitually) In fact, non-observable actions can be used in the continuous, too, but that changes the meaning of the verb a little. I’m silly. (Always) I’m being silly. (Now. So ‘being’ takes on the meaning of ‘acting’) I want an orange juice. (Now) I’m wanting an orange juice. (I’m visibly demonstrating that I want an orange juice. This use is more advanced.)