Maybe this can be answered in a couple of ways, with reference to the different people there and the rather complex history of the region.

So, the map of the space between India and China where we see Myanmar, Thailand, Cambodia, Laos and Vietnam, has changed quite a lot over the past couple of thousand years - but was never unified as one kingdom. The region’s borders indicate the areas where different ethnic groups who maintain different languages and customs reside. For instance the Khmer in Cambodia have been living there for perhaps 10,000 years - but for at least 2000. The different people’s relative distance to the two massive civilisations of China and India to the east and west played a large role in the development of unique and varied cultures - Vietnam being closer to China has more Confucian influence, whereas Burma and Cambodia are more Indianized.

In fact Thailand and the kingdoms/ethnic groups that preceded it were a feature of mainland Southeast Asia later than some of their neighbours were, the Sukhothai Kingdoms rise to prominence around the 12-14th centuries and were primarily Theravada Buddhist, along with Burma and eventually Cambodia. Vietnam is not Theravada. Most of the people in the region consider these neighbouring countries with suspicion related to ethnic differences or previous entanglements. Cambodian history from around the 14th century is characterised by fears of dominion from either Vietnam or the Thai.

So there was never one completely dominant kingdom in the region. Even the Khmer Empire at its height did not conquer the Western kingdoms that would become Vietnam.

When we look to more modern contexts to find answers to why the map looks like it does today, the influence of colonial powers from the 17th century onward is important. Again the region was never controlled by one power: Britain had Burma and Thailand, Vietnam Laos and Cambodia were French assets. With reference to that ‘southern flat’ you mention, the southern part of Vietnam was once part of Cambodia and is known as Kampuchea Krom to many Cambodians today - a territory they were quite keen to get back during the Khmer Rouge years. So yes, that is easily traversed and was previously just one territory.

There were possible plans of the Vietnamese to have a kind of incorporated Communist state in Indochina following their victory over the US, but the relationship between the Khmer Rouge and the Vietnamese was incredibly strained - and the influence of China in supporting the Khmers (and preventing a new bloc supported by the USSR on their border) can be gleaned from the war fought between Democratic Kampuchea and Vietnam in 1977-79.

Since the second half of the 20th century was characterised by the right of national groups to have their own countries, self determination, it makes sense that there were not further efforts to create one state that incorporated all of these disparate ethnic groups, languages and customs.

In fact your question could easily be answered with reference to the - in some cases politically incorrect term - ‘Indochina’. This word reflects a western colonial attitude toward the region which assumes more uniformity in the region than the rich cultures of each country would suggest. The word can be used to refer to the bit of Asia between India and China but it is more commonly referred to as mainland Southeast Asia - rather than Indochina. The key to answering your question is the differences in the people there that make it impossible to say ‘an indochinese person’, such a group or person doesn’t and hasn’t existed.

To summarise, the diversity of language and culture between different ethnic groups in Southeast Asia as well as the history of rivalries precludes any serious attempts to unite these different groups into one territory. The map of the region has changed a lot in the past two thousand or so years but was never a united kingdom nor was it controlled at any one time by one colonial power.

Edit - I actually read the question in bed and didn’t quite notice the amount of emphasis you put on the geography of the region so sorry that I don’t refer to that much, and maybe that can be explained by the way in which the map and territory of each state has changed and morphed a fair bit over the years.