Why am I slowly losing air in both my left side tires?

I have a Honda Prelude 1999.

I already took my car to a tire shop. They dunked the 2 tires and said both were fine.

If they didn't totally submerge the tires it could be the valve. To save yourself money you can check the tires yourself. Mix and few drops of dish detergent and water in a spray bottle. Spray the tires and a leak will show up as bubbles.

Change the valve in the valve stem and see if it helps.

Power to the people.

Might be a leaky valve. If there aren't any major leaks I'd just check the pressure regularly and keep them inflated.

Chk/Rep Filling Valves.

Because somewhere there's a leak of air-pressure, no doubt about that, because air doesn't dissappear into nowhere within the tire, its going somewhere and to find where the air is going, dunk the tire again by the own hands and see then with the own eyes where tiny bubbles come out from the dunked tire as the bubbles are the exact place where the tires are loosing air-pressure. When they dunked the tires they didn't look close enough to see, to then claim the tires were fine.

how to dunk the tires, get a wide plastic wash-tub from the dollar store, lay the tire with the side into it and fill it with clear water, the tire should be completly under water, clear water to see the bubbles better, flushing the tire before would help to get the dirt off to not murk the water in the plastic tub, to see where exactly even tiny air-bubbles come out from the tire. Flipp the tire upside down in the plastic tub, there could be a leak on the other side too.
the advantage of laying the tire with the side into the water (as i wrote) but not with the tread towards the ground is, that turning the tire would stear the water and tiny bubbles can't be seen then (at least not that easily) but when the tire lays completly under water with the wide side up, then the water will stand still and very tiny bubble will be visible.

use white oil color to make a white dot on the bubble and take a picture of it, to identify the damaged spot for a repair.

[a possibiliy is, that some jokestar lets the air out of the tires, if the car is parked on the street, only a thought but who knows.]

if the tires are not replaced, at least put the two tires that loose air on the back, that is more secure than when they are on one side (whether left or right). At the usual highway speed that won't be to dangerous, if the speed is going much higher, the little damage exposed to increasing centrifuga force by increasing speed, can make even a little hole rip open and high speed are deadly for the driver, passangers and random people around.

It takes a special eye and patience to find a slow leak. The slow leaks usually happen around the valve or rim.

They're obviously NOT fine.

A bird or similar is deflating your tyres to keep cool in the escaping draught. This is not unique. Use only METAL valve caps, they seal and protect the valve.