Why do my people get unhappy with farms?

More population equals lower morale. Offset the morale drop with "happy" buildings and morale resources. Also if you start pushing population up to or beyond 20B on a planet you'll get a steeper morale dropoff than you do at lower pops.

Becasue of overpopulation?

After ~18-20 billion ppl morale drops exponentially so I never bother growing my worlds above this as it only affects growth rate on other worlds when trying to maintain 100% happiness.

The only two reasons why you want a huge pop planet would be

a) to get really high scores in the metaverse or

to make production/transport planet, since the late game somewhat requires you to have planets which can both quickly produce military transports and at the same time hold a large number of citizens for you to recruit.

and

c) higher tax income

I usually add pop to my world when they hit max pop and high morale. I'd rather have high pop then high morale- as the morale boost only helps pop growth- which is 0 when you're at max pop anyways...

i usually have a few high pop worlds, and the rest with only 1 farm. if i can get all the morale goods and some morale bases, i don't even need a morale building on these planets.

what i usually do is gradually increase my tax rate as i get additional morale bonuses. i try to keep everyone at 100% morale except my money making worlds. if i can keep them in the yellow, it's cool.

i'm playing a game now where the same world got hit twice by the precursor artifact that doubles the base quality event. it started as a 9, and it's a 47 now in my control. i took it from the Korx who'd thankfully built their economic capital on it. it has 5 farms and at least 8 or so virtual reality centers, but it's at about 85% morale, and it's over 38b people climing towards the max of 41. the income from that planet alone is about 5k/year.

and c) higher tax income

Exactly. The econ buildings are multipliers, so one world with 18b people and 3 banks will bring in more money than 3 worlds that each have 6b people and 1 bank. It'll also get up and running quicker since you only need one world's worth of factories instead of 3.

Specializing so that the big worlds bring in mountains of cash that fund the factories and research centers on the smaller ones seems to work out pretty well...at least on the settings I currently play at (common worlds, common habitable worlds, Large size).

I wish the research tree for farming could be expanded a bit more. Due to the limitations of population and morale, it seems like a limited affair to research. I'd love some of the higher techs to add a small happiness or influence bonus for some of the higher advanced techs. This could be represented as not only improved production efficiency, but maybe a broader diet available to the people on a planet. Hence, folks are 'happier' with a large variety of foods, or the world's influence increases as it is a tourist hit known for their excellent 'cuisine'.

i.e. nicer foods tech

That's essentially what the Ultra Spices are.