Because Harry is filled with a deep-seated self-loathing and thinks that if he ever tries to use Magic to make his life less terrible by even the smallest of degrees he'll either turn into a full-blown Supervillain instantly or the universe will reach down and squish him like a bug for daring to live any other way then in abject suffering just above the poverty line.
TLDR: Because Harry's an idiot.
This answer also covers other questions, such as:
"Why does Harry never again make any of those OP potions that solve half his problems"
"Why doesn't Harry ever find out about things like freaking Vampires until post-SF, even though he's had Bob for years"
"Why does Harry never look into healing Magic on any level, despite his suicidal lifestyle"
"Why doesn't Harry work on his water Magic, since that apparently counters a Wizards weakness to flowing water"
And so on and so on (Seriously, Harry is terrible at Wizarding)
Jim has addressed the first point: A wizard only has so much power to invest in their magical gizmos. At this point, Harry is rocking things like his staff, jacket, shield bracelet, and blasting rod (essential tools for amping up his wizardry) and so does not have the magical umph to spare.
Additionally, as OP as those potions were, they nearly got him killed, and are one-use only.
On the healing magic front: It is -incredibly- difficult. Listens To Wind has built his life around it, and even he cannot cure everything. Harry, who has less experience and less innate talent for slow, low watt magic, is unlikely to do anything meaningful with it.
Water Magic: Maybe you missed it, but Harry has been throwing around an awful lot of ice lately. Ice isn't, inherently, water magic, but Harry is still working within that realm.