Simply put, because you are most likely not average. In all honesty almost no one is, it is kind of the point of an “average”; you take all the data, add it up, divide by how much there was and tada! A figure somewhere around the middle that suits everyone kind of, some better than others and some hardly at all.
There is a little bit of differentiation, clothing sizes pop us into bands where we hope that because we have a 38” bust the shoulders, arms and waist will all fit the average proportions. Again, chances are they don’t but in most cases they are kind of close enough to live with.
So, for me, if I take the typical sizing for my bust measurement (because that is what many patterns use as the measurement to base your size choice on), here’s a few of the ways I differ from the averages that are going to mean a standard sweater will probably not fit (side note- these are literally based on how my numbers differ from the published averages, no body criticism here):
I am short - shoulder to waist/hips/hem are likely to be too long
With wide shoulders - sleeves may pinch into the underarm if the shoulders are too narrow.
Larger than average bicep measurement (this is usually the killer for commercial clothes) - sleeves are often snug in a way I don't like, this can cause underarm pinching too.
Relatively small waist - need to bring the fabric in to avoid it looking too baggy, but prefer it not too tight over the tum!
Your list will be different, but most of us vary from the averages in some way. It will change over time as well, bodies do that.
The advantage of making our own is that we can adjust the patterns to get a better fit.
So, coming up, some posts that will hopefully help you to get a better fit from your knits (sweaters to start with, but applies to others). We will talk measuring, gauge, ease and adjustments for fabulous fit.
Have a think about what you have found fits well in patterns you have made, and what you might like to adjust the next time you are working on a project.
Pattern shown "Ink" vest, worked from the shoulders down with plenty of armhole room, all over textured rib and a slightly cropped length.