The reason why primary and excess coverage is split across two policies is generally insurance regulation.
Primary insurance coverage is heavily regulated on a state level. Companies carrying primary coverage must be licensed and regulated at each state they sell policies in. Additionally, depending on the state and type of insurance, there are further regulations because regulators see this as a public necessity.
For example, some states have auto insurance on a no-fault basis. Some states cap the maximum cost of an auto policy. Features like "accident forgiveness" may be banned. Some states even tax insurance. This can cause the cost and details of insurance to vary widely.
As you have noticed, most states put a $300,000 or $1 Million liability limit on automobile primary coverage. Excess insurance is regulated differently.
This is for various reasons: A main reason is that people or companies who need excess coverage have the resources to know what they're buying, they don't need state governments to be setting rules.
When excess insurance is regulated, regulators don't see it as such a necessity and take a more hands-off approach. For example, excess coverage may exclude some risks that primary coverage can't. The carrier may have a greater ability to drop somebody or exclude a particular driver.
Sometimes the carrier is licensed in one state but sells in many others. This is because that the market is much smaller for this type of insurance and a nationwide market is necessary for efficiency. For some lines of insurance like what an airline buys, the coverage might be so large that you need international resources. Finally, at the large amounts where excess kicks in, loss rates tend to be more similar across states.
If you really want to know more, here's 100 pages on excess insurance regulation.