Because they can sell CBU units before that to gauge the market.
An car assembly plant isn't something you can carry in your school lunchbox. An entire manufacturing plant needs to be built and it takes years. Along with that, they are planning to build the charging infrastructure in major parts of the country as well. This all takes time.
Why do the drama of launching when nothing is ready until 2026.
Also, Mega / HUBCO is publicly listed company on the stock exchange. They're legally required to disclose any major investments and business deals to their shareholders and the stock market itself. Since they had to disclose what their plans anyways, going a step further and creating some hype in the market doesn't hurt.