Some of the other features of a shopping cart
1. Integration with a checkout system (Single page Vs Multi Page) - this is itself a demon in itself.
2. Bells and Whistles on the cart itself like tax & shipping cost estimator
I guess when you say you can create a shopping cart in 15 mins you are not keeping in mind any checkout flow integration or estimators or much control with items in the cart e.g. changing sku options like color etc.
There are some open source systems like Magento which can be looked at too.
I was involved with an ecommerce startup that built all of this from scratch to control everything legally and then sell it, and quite honestly, it is an absolutely incredible amount of work.
Usually, it is easier to find software that's already been written to meet your needs unless you're trying to create a unique value proposition in some way.
If you're building a standard e-commerce site, it might make sense to use some software package that's prebuilt with what you need.
Either way, if you think you can do it in 15 minutes and want to build it, have fun with it and do it yourself.
There are a lot of features an simple online shop might want to use. Cart is used as a general term