Could you elaborate on why it's a good idea to start a business in a recession?

Reduced competition. Fewer other businesses are trying to start up at the same time and existing companies have less money to spend to compete against you.

Suppliers will also be eager for new business, same with landlords if you have a physical store. Also easier to hire.

Wouldn’t there also be reduced market/customer demand though as well? Guess it depends on the startup and business model, but could see a challenge with actually selling whatever it is your business is providing (e.g., services, product, etc.).

The economy won’t stay depressed; you have time during the recession to build up, get ready while there is less competition or your competition is very weak. When the market moves up again, you can go full in. It is just a lot cheaper to do the startup part of the startup (building the product, market research, hiring, or, marketing, sales, getting first clients, etc) part of the startup. After the recession, you are the one ready, the ‘old ones’ are licking their wounds (they lost trust by alienating clients with sudden price hikes or policy changes and alienated employees by firing large portions of them etc) or gone.

As I commented in another thread, it's because it's harder to get a job and get a raise. I took a full time job because it's where the money is, but back when the tech industry wasn't as hot, it's easier to make money as a business or contracting.

Because investing in everything else is suboptimal and the new business can be ready to make money when the market inevitably returns to strength.

How long does it take on average for a new business to become profitable? Years if I'm not mistaken. The markets won't stay depressed for that long.