Before you can sell a new product you have to make sure it complies with all the relevant legislation (such as CE marking).
Unexpected time and cost
The time and monetary cost of this can be unexpected for a young designer. And since it comes towards the end of a project, many costly design decisions may have already been made which make compliance even more of a headache.
Eliminate it where possible
Lets imagine you’re designing a new electronics product. You could design your own power supply, or use one off-the-shelf which is already certified.
Whatever gains you make from designing your own power supply (perhaps lower unit cost, better performance, space saving etc.) can almost all be wiped out by the cost of testing and proving compliance of your new design.
If you want to get a product to market quickly then do as much as you can with off the shelf parts. This is the role of a lot of design and engineering nowadays - systems integration, being able to skilfully knit together different pieces of software and hardware to create good solutions.