Because there are domestic rights and privacy laws that have to be abided by that countries don't follow abroad. Also, foreign intelligence is very different tactics/skill-set from domestic. Tactics used abroad to get information wouldn't necessarily work, and they may prevent successful prosecution of crimes because they violated rights, didn't properly collect evidence, etc.

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Domestic intelligence obtains evidence legally to use in court although may use illegal means at times to know what evidence to collect. It's much closer to a police force. Foreign intelligence only cares about not getting caught; everything it does is illegal. Comingling domestic and foreign intellifence could produce complications because the illegal foreign activity could be exposed in court.

There's two main branches of domestic intelligence, pure domestic intelligence, such as against domestic terrorist groups, and counter-espionage, which fights foreign intelligence agencies. Those two skillsets by themselves are very different from foreign intelligence. It's like asking why a quarterback doesn't play safety.

From open source information in books about the topic, an example of the issue would be foreign intelligence bribing or blackmailing a source, and then having to internally weigh the credibility of the information.

Opposed to many countries having citizens rights about trial and facing accusers, so information gained because they threatened someone about exposing an affair, is not considered okay in court.
Foreign intel does not need to convince a jury with a judge and defense attorney. Credible information can be "Well we bribed a food company dock worker and know their ships just loaded enough food for 3 months."

It's one of the issues that is cited in the intelligence failure before 9/11.
Foreign intelligence didn't want to work with Domestic intelligence because domestic intelligence would need to explain in court where the information came from. Also because they didn't trust them to maintain security on the info. Also because there is a higher duty to act domestically. Foreign intel may want follow something and see where it goes to work the entire network, while domestic might want to make an arrest. Domestic didn't want to work with foreign for the inverse reasons. Getting info from something that would violate rights domestically means info is unusable in court. Foreign side wouldn't elaborate on sources either which is not enough for legal proceedings.

Another dimension to this is that intelligence agencies, by their very nature, are very difficult to have proper oversight and regulation of. So it may be in a nation’s best interest to have multiple separate agencies that don’t really compete with each other and have independent chains of command so that one agency doesn’t unilaterally control everything and it’s more difficult to compromise your intelligence network. See how in the US the NSA handles mostly electronic and signals intelligence while the CIA deals more with human agents and spies, and the NRO handles space assets and satellite based intelligence that it provides to those agencies and the many others in the US intel network.

Internal Intelligence is used to keep domestic threats at bay - could be a separatist movement, domestic terror plots or anything that happens internally that can destabilise the govt.

External intelligence is to keep the country safer from external threats like another country or factions posing as a threat to the country’s citizens or monitoring the global events ans Happenings and assessing if they are in tandem with the country’s international interests.