One reason is; the course related job taken on but not fulfilling hence they go ahead and 'follow their bliss'- doing something else that gives them a deeper feeling of achievement, and fetches good money of course. For example, the preschool/creche I worked last year is owned and run by a woman who studied Law and got called to the Bar. I know two women who studied Pharmacy yet one bakes cakes and deserts for large parties/special events and runs a Catering School; the other is a consultant at FITC (Financial Institute Training Center) here in Lagos. A friend studied Chemistry and works at Chevron yet her real calling is making organic cosmetics and aromatherapy oils.
Another reason is the scarcity of jobs in the job market (NO VACANCY) or not meeting the job requirements which drives them into doing something else to put food on the table. These days, graduates who studied Accountancy are in classrooms teaching Mathematics because they can't get work in Banks where they belong , Architecture grads teaching Fine Art, English Language & Creative Arts students in the Marketing Department of any company. Those who teach are known to sell stuff like handbags, shoes and clothes to earn more money.
And there's the most common reason; there are those who were unable to go to university or finish secondary school because of poverty and they are the real hustlers because they have to put their natural or learned skills to use or just go into a business based on demand. They are the vulcanizers at the side of most roads waiting to fix flat tires.
Some are those who get a few desktop computers and a good internet service and run cybercafes; adding to the business by also selling snacks, drinks and computer accessories to the customers who come to 'browse' the Net.
They are the market women who sell anything from tomatoes, pepper and onions to fried akara or run road side canteens. Some have even started selling home made dish washing soap- which is sweet smelling and very foamy and of course cheap (50 to 100 naira depending on the size of the bottle) and other stuff like seasoning, dried fish, fruits and bottles of kerosine.
Those who mostly prefer to use skills to make money go about it in the following ways.
Selling Recharge Cards/ Phone Operation: Very competitive since everyday people need to reload their phone credit so demand for recharge cards is very high. If you don't have a cell phone, you can pay to make a call. You can find recharge card vendors and phone operators' kiosks just about anywhere; it's only a few who offer other wares besides cards and calls; some sell candy, snacks or new cell phones and Sim cards to anyone who can afford them. A friend of mine (who was unable to follow her dream of studying Nursing) sells recharge cards and bakes meat pies, doughnuts for sale. Not only that, she also makes popcorn and Jollof rice, Fried rice topped with fried turkey and turkey gizzard- fetching her more money to live on.
Also competitive and takes a lot of skill. Tailors usually prefer to attract customers by sewing clothes of original design and sell them first before taking commissioned work by them. Location of their shops is also important.
Hairdressing: Nigerian women always have to look their best hair wise so there are unlimited hair stylists more than ready to do the job- from shops, near car parks and via home services.
In short, everyone just tries their hand on anything they can think of (legit of course) to earn an income. Be it in the form of preference or necessity- they always manage to keep head above water.