Well thats because you are not a full stack engineer. If you were you would understand. Its pretty simple. You have backend , you have front end , you have database. Full stack enginner has experience and implements features across all these technologies. A bit of dev ops environment knowledge is probably also good because you have to worry about the connection between all these environments in order for you to do your work.

Sure there is a quality trade off, being jack of all trades but this industry is very elastic. You can have a great frontend engineer with only 2 years experience and an ok one with 6 years experience. Time in field doesnt really gauge skill well. Its more about the challenges one takes on and the variety of problems they solve with their code.