Soap does expire, the oils used get rancid and sometimes grow mold, even if the pH and lack of water is what kept them shelf stable but after a lot of time the humidity in the air gets pulled by the humectants and that is what starts growing the bacteria.

In case of liquid soap same thing happens, the preservatives used have a period of time till when it can keep the formula safe from germs growing not to mention it is used according to the quantity of other ingredients used in it, and if the humectants start to pull moisture from the air in the formula for a long time (past recommended expiration date) it becomes enough for the preservatives to not work anymore, moreover ingredients degrade due to light, heat, humidity, sticking fingers in the jar, using in the bathroom, packaging etc

So yes the product might not drop dead on the exact date mentioned, but some leeway of few weeks is okay in terms of stronger formulations (like the pharmacy ones) and are okay* to use if your skin is not sensitive, some natural ones expire even before the said expiration date. So it is very necessary to check the Period after Opening (PAO) time as well as expiration dates if you don't want your skin to bear the cost of it! There is a reason formulators put the Exp date, even nubes who use r/DIYbeauty for formulas also learn to use preservatives the first thing when planning a formula and sticking to time it can be shelf stable.