Rivian Automotive (RIVN 7.80%) is about to enter one of the most exciting growth spurts in its history. It plans to launch a new discounted model in as little as six months, with two more new models to follow soon after. These launches could easily double or even triple the company's revenue over the coming years.

That has investors excited and the stock price on the rise. Think it's too late to invest? Think again. There's one reason Rivian shares remain surprisingly cheap.

Rivian doesn't have this exciting business segment

Other electric vehicle stocks like Tesla and Lucid Group trade at 6 to 13 times trailing sales. Rivian, meanwhile, trades at just 2.6 times trailing sales. Why the big discount?

Growth is a big part of the story. Lucid is expected to grow its sales by 60% this fiscal year, while Rivian's sales are expected to grow by just 6.5%. Tesla is the outlier. Analysts expect its sales to fall by around 5% this year. Why then does Tesla stock remain extremely expensive? It turns out that both Lucid and Tesla have a growth opportunity that Rivian hasn't pursued very aggressively: robotaxis.

A person and a dog stand near a charging electric vehicle.

Image source: Getty Images.

Robotaxis are expected to be a huge market. Some analysts think the global opportunity could eventually be worth up to $10 trillion. Recent robotaxi announcements from both Lucid and Tesla, therefore, have given investors plenty of optimism for long-term growth potential.

Rivian has been surprisingly quiet on the robotaxi front. One of its board members even recently criticized Tesla's efforts, stressing that the service has a long way to go to be truly autonomous. A lack of robotaxi upside is likely a strong reason why Rivian shares are priced at such a relative discount to the competition.

The robotaxi opportunity is still very early stages, however. Rivian's new models, meanwhile, are near-term opportunities. We should see growth rates pick up considerably next year, allowing investors to take advantage of today's discounted valuation. Rivian simply doesn't need robotaxis to grow considerably in 2025.