If a can of peas or any tightly sealed can of food in it is "NEVER" opened why is there a expectation date?

Manufacturers have an interest in you thinking their food looks and taste good. They put a "best by" date on the can (not an "expectation" date, or even an "expiration" date) so that you will consume the contents while they can still guarantee optimal taste, appearance and nutrition.

As long as the can isn't damaged, canned goods are safe to eat for many years. However, the texture, color and nutrition may deteriorate.

What is 'a [sic] expectation date'? Do you mean expiry date?

Most foods eventually expire, ie. they 'go bad' in some way or another, no matter how they are preserved. The expiry date is there to give you some indication (albeit fairly arbitrary) of how old the product is and when it might be time to discard it.

The expiry date can be either 'use by' (= you are advised not to use the product after the expiry date), 'best before' (= will be OK after the date, but the taste, appearance, etc. may start to deteriorate), or 'sell by' (= instruction to the retailer to remove the product from display after that).

All that said, the manufacturer has no real way of knowing how long the food lasts, so they take a guess and then reduce that by some safety margin. It's of course in the manufacturer's interests to keep the shelf life as short as possible, as this a) means less risk of the food going bad before the expiry date, and b) more likely that the retailer or the end consumer will throw it away and buy a new product.

Lizzie is right. It's more of a quality thing than a safety thing. Also, expiration dates are required on every edible item in some states of the US (and perhaps other countries), and expiration dates help stores manage their inventory (they can tell what the older items are by the expiration date on the can) and rotate stock accordingly.

there are possibles that certain small fungal/ bacterial items can get into your food through the can itself. so people at the company test it and put a date on the can just in case a few years go by and you decide to eat a can of peas or whatever you want: that expired maybe a few months or a year later than the can was expected to be open.

To make you throw out the can with an expired date, but perfectly good contents, and buy another one that you don't need to, hopefully to do so again ... and again.

after all they have a shelf life!!!!!!!!!!!