That is stupid.
There s no point in playing with less than a full party.
Sure there is. More of a challenge. Easier time using stealth to its fullest. Going solo would be like going Rambo on the goblins using stealth and intelligence to win. Which is fun.
If going solo also over-leveled you for the content it'd remove the challenge of playing solo in the first place.
Meanwhile if you get tired of playing solo you can easily go back to camp, grab some followers, and head out.
I like it this way. It is flexible. And it means I can freely switch out companions and see who I like without worrying about missing out on exp.