There are several reasons why we don't commonly see 1200hp 6-liter naturally aspirated engines in cars:
Practicality and Efficiency: Higher power outputs in cars often come with trade-offs in terms of practicality and efficiency. While sports motorcycles prioritize power-to-weight ratio and agility, cars need to consider factors like fuel economy, emissions, and overall drivability. A 1200hp engine would consume a significant amount of fuel, and the power might be difficult to manage effectively in a car.
Cost and Affordability: Developing and manufacturing a high-performance engine is a costly process. Creating a 6-liter engine capable of producing 1200hp would require expensive materials, precision engineering, and advanced technology. The resulting car would likely be prohibitively expensive for most consumers, limiting its market appeal.
Safety and Regulations: High-power engines require a robust chassis, suspension, and braking system to handle the increased performance safely. Meeting safety standards and regulations becomes more challenging as power levels rise. Additionally, many countries have speed limits and regulations that restrict the full utilization of such powerful engines.
Market Demand: While there is a niche market for extremely high-performance cars, the demand for such vehicles is relatively small compared to more practical, everyday cars. Automakers need to consider market demand and cater to a broader range of consumers who prioritize factors like comfort, practicality, and affordability.
Advancements in Turbocharging and Hybridization: In recent years, automotive engineers have focused on developing turbocharged engines and hybrid powertrains to achieve high power outputs while maintaining efficiency. These technologies allow for better power delivery, improved fuel economy, and reduced emissions, making them more appealing for a wider range of car buyers.
It's important to note that there are indeed cars with extremely high power outputs, but they often rely on forced induction (such as turbocharging or supercharging) or hybrid powertrains to achieve those levels. Purely naturally aspirated engines with such high power outputs are less common due to the reasons mentioned above.