What are the incentives of getting a job over other money-making activities?

I've noticed my Sims seem to earn a lot more money when gardening than they do at their day jobs. After crunching the numbers, I found out this wasn't just an illusion - depending on the crop of choice, a Sim can earn money two to three times faster with gardening than they can at an entry-level day job with roughly the same amount of player effort. If the player has time to devote to constantly refreshing crops that have shorter growing cycles, much higher rates are easily possible!

Of course, the Sims do advance in their careers and so can earn more money with their day jobs. But, by the time that's achieved, the player can just as quickly have reached a level where even higher-yield low-effort crops are available.

So, where's the real incentive for the Sims to get a job? Is it just a matter of player preference and role-play, or are there other benefits in the game's mechanics that I'm just not seeing yet?

You are correct in that past updates of the game gardening, baking and certain hobbies were much more efficient in payout rate than careers. However, with the most current update (version 5.12.0) after all of the milestone quests and challenges, careers finally make as much, if not more, money. Here are the numbers to back this up.

Here are the rates for gardening and baking, sorted by highest to lowest rate per minute. This is assuming you purchase seeds and ingredients at the grocery store to procure the lowest price.

Gardening
Bell Pepper: §8.00 / minute
Carrots: §5.80 / minute
Corn: §4.50 / minute
Watermelons: §2.53 / minute
Potato: §2.17 / minute
Zucchini: §1.75 / minute
Onions: §1.61 / minute
Beans: §1.36 / minute
Hamburger: §0.63 / minute
Garlic: §0.36 / minute
Pumpkin: §0.35 / minute
Chilli Pepper: §0.18 / minute

Baking
Hot Cross Buns: §1.50 / minute
Cookies: §1.00 / minute
Heart-Shaped Chocolates: §0.75 / minute
Pancakes: §0.60 / minute
Apple Pie: §0.45 / minute
Muffins: §0.43 / minute
Choc Pudding: §0.25 / minute
Cheesecake: §0.17 / minute
Gingerbread Sims: §0.14 / minute
Caramel Slice: §0.12 / minute
Croissants: §0.10 / minute
Danish: §0.10 / minute
Turkey: §0.08 / minute
Pumpkin Pie: §0.03 / minute

This doesn't include transgenic crops or bakery items, which have a failure rate of somewhere around 50%, so in the long run those don't necessarily pay off. The quickest crops and baked goods in general have the best rate.

At the highest level (5), here are the equivalent rates for working at a career, sorted by highest to lowest pay rate.

Careers
Actor: (5 hours) §6.20 / minute
Real Estate Agent: (5 hours) §5.08 / minute
Athlete: (6 hours) §5.00 / minute
Musician: (7 hours) §4.17 / minute
Teacher: (5 hours) §3.48 / minute
Politician: (6 hours) §3.39 / minute
Artist: (7 hours) §3.00 / minute
Scientist: (8 hours) §3.00 / minute
Fire Fighter: (9 hours) §2.78 / minute

The added benefit to having a career comes when there are particular goals that are career related (such as, send 5 Sims to work, or start a career as a politician) or in order to fulfill life-dreams for orbs. In the case of orbs, career life-dreams are much more efficient than many of the other life-dreams, and still enable your Sim to have time to partake in other hobbies and/or gardening or baking when they're not at work.

As you can see, the careers aren’t necessarily much more time-efficient than gardening, however, unless you want to spend 30-second intervals gardening, you are better off having your sims go to a career. They will also make more money at a career while you’re sleeping than doing a several hour gardening task.

All of this information is subject to change with future game updates.