Why do cyclists use padded shorts instead of padded saddle? It seems like putting padding on the saddle would be more optimal. Doubly so for triathlon.
Padded seats tend to have more padding than used at any given time. This pushes the other 'extra' padding into the soft tissues. This causes numbness and discomfort over time.
So slim hard seats are actually more comfortable over time, if they are the right size. You need to make sure your sit bones (ischial tuberosity) are well situated. The sit bones of your hips are designed to support weight for long periods of time, where a plush seat disperses your weight to muscles and other areas.
Putting the padding in the shorts means that the padding will always be exactly where you need it. Most people tend to change their position on the bike a bit, specially when riding longer tours. With the padded saddles, the padding won't change when you alter your position, which can result in blisters or sores.
It's not really "padding", although it does offer a cushion. It's a chamois, which part of its purpose is to absorb moisture and wick it away from your skin which helps reduce friction. This is aided by creams that condition the chamois.
and +1 to @Matt Adams answer.
It's never a good idea to rely on the saddle for padding. The reason is simple. The sit-bones will sink into the softest saddle and therefore put extra pressure on other parts of the male anatomy where you end up feeling as if your 'privates' dropped off on the road a few miles back. You should also be 'fitted' for a seat so that you get the right width of saddle which suites you.
The chamois is a crutch for an ill fitting saddle and bike set up. Chamois are "gaskets". They hold moisture, bunch up, grip soft skin, causing friction that heats and causes burns, etc. They get worse with slight aging. If the saddle is the right shape for your groin, there should be no "gasket" introduced to the groin. My personal preference is a leather saddle (I use the B17 Brooks) that "gives", breathes, wicks away moisture and stays cool. I use a chamois-less Lycra short that ventilates immediately. With over 50 years of riding several thousand miles per year, This is what works for me. I have a garage full of saddles, many that I've modified. Cheap, ludicrously expensive, you name it. My next to best favorite (BTW) is the Selle Italia SLR superflow 130mm. Big cut-out, very thin padding, flat front to back, and narrow. Again, I use no chamois. 100 mile rides are easy. Once I wear out the Selle as a "now and then saddle", I'll just swap the B-17 around on the bike of choice for the day. It won't wear out in my lifetime. My original was built in the mid 70's and the replacement was built in 2013.
I would bet the answer is because of tradition. Long ago, seats were hard leather and riders were susceptible to stress risers and pressure points on the seat that would cause irritation. So they skinned an animal for it's durable padding and put them in their shorts. Then came injection molded seats, but riders were used to padded shorts. Then came fancy short pads so they didn't need to use animal hide anymore. Companies played around with different types of pads to see what sold, and they found that more padding sells better. FYI: More padding in shorts is bad. I design saddles and I disagree with the designs. I try to make a saddle with just the right amount of padding. These bulky short pads just screw up my design and put pressure where it's not intended. PS FYI: Bicycle gloves have the same issue. More padding hasn't been shown to help anything. The issues are more complex than just adding more padding.