The two main reasons for this is
1. Power companies, unless they are part of a cooperative, are private businesses and therefore they don't connect their lines with another.
2. Different companies may have lines operating at different voltages so unless the companies want to share the expense of installing transformers the lines can't be tied together.
Here in Florida, the largest power company, Florida Power and Light, is a private company but they do buy and sell electricity with other companies including those in Georgia.
FPL is also one of the few private power companies that owns their own generating plants.
Smaller companies usually have to buy their electricity from a larger firm like FPL.
That makes sense. The instances I've seen where the lines tie together or share poles is with NHE co-op and Eversource, the larger firms here. I'm not sure but I think all of the companies here own their generators.