The canning process seeks to destroy bacteria (and fungal spores) within the product, and then seal that product within an environment to which bacteria (and fungal spores) have no access. The process is remarkably efficient in achieving its aims, but not perfect. That means that some decomposition of the food (whether by bacteria or by fungal growths, such as moulds) will eventually occur.
In general, the bacteria and fungal spores which survive the canning process (and go on to start decomposition of the food) are not those one which are dangerous to health. (e.g. salmonella will not survive canning unless there is a fault with the process). So, while the quality of the food will degrade over time, health risks remain minimal.
I've just taken a look at the cans in my kitchen cupboard. They all have 'best before' dates on them (which refer to the possible degradation of the taste and 'quality' of foods) but I can find none that have 'use by' dates (which refer to potential health risks).
Chris