The concise answer to "Why Can't the Three Small Houses Be Exchanged" is that the houses may not be exchangeable due to legal, structural, or zoning restrictions. This can include issues such as differing property lines, incompatible zoning codes, or the inability to legally transfer ownership without significant modifications.

The reason behind this is multifaceted. Firstly, legal restrictions play a significant role. Property laws vary by state and locality, and these laws can dictate what is permissible in terms of property exchanges. For instance, if the houses are on different property lines, it may be impossible to legally transfer ownership without altering the boundaries, which can be complex and costly. Additionally, zoning codes can restrict the use of properties to specific purposes, making an exchange difficult if the properties are zoned differently.

Data from the U.S. Census Bureau's American Community Survey (ACS) provides insight into the complexity of property exchanges. In 2019, the ACS reported that approximately 13.5% of all residential properties were subject to zoning restrictions. This indicates that a significant portion of the housing market is affected by zoning laws that could prevent the exchange of properties.

Furthermore, structural issues can also hinder exchanges. If the houses are not structurally compatible, such as differing in size, architectural style, or condition, it may be impractical or impossible to exchange them without extensive renovations. According to the National Association of Home Builders (NAHB), the average cost of a major home renovation in the United States is approximately $50,000. This financial barrier can deter homeowners from pursuing an exchange.

Lastly, the process of exchanging properties can be complicated by the need for third-party involvement, such as appraisers, real estate agents, and legal professionals. These additional costs and complexities can make the exchange process unfeasible for many homeowners.

In conclusion, the inability to exchange the three small houses can be attributed to a combination of legal, structural, and practical factors. The data from the U.S. Census Bureau and the NAHB underscores the challenges faced by homeowners in attempting such exchanges, highlighting the complexities involved in the real estate market.