Applies to touring car & superbikes (please have a look at superstock of you have the time) as well. Also they're a bit more exciting to watch because they aren't heavily dependant on aero & downforce compares to single seaters/prototypes (doesn't apply to DTM because they have huge amount of downforce that is equivalent to single seaters).
Fundamentally, owning cars incur expenses. You want your car to be faster & more powerful, handles & stops better, you'll have to pay more. When you drive hard & harder, you'll have to pay more frequently for consumables/wear & tear. Same goes to racing cars, even at club levels (but with several regulations to keep costs low).
Extra reading:
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This will never happen today, but in the 90s people would go to tracks just to watch bunch of Civics & Sagas racing. Sagas are for beginners (with a mix of pros) while Civics are for pros. Even Alex Yoong & his dad raced Saga back then. And there's a handful of richfags with their Porsches. They'd even watch kapcais (we still have Cub Prix) & 2-strokers racing but that's another story
Don't read if you're an "F1 fan";
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F1 did nothing but killed local motorsports scene. Govt, private sectors & medias turned their attentions away from local motorsports & this hurted local motorsports. They gained very little from F1. Tourism excuses doesn't really make sense after few years of hosting F1 (don't be fooled by Tun M's propaganda). Those millions paid to Bernie & co could've been used to build more tracks. More tracks = healthy motorsport industry/scene (the ones that benefited the most are tuners, builders, fabricators etc. Booming business = good for economy etc). Instead they demolished a great track that's accessible to anyone & another one might be demolished soon. Look at Japan & Thailand for example. Japan has a long estabilished local scene (anyone who watched Best Motoring & Video Option knows that they have a lot of smaller tracks apart from bigger tracks like Suzuka, Fuji, Motegi etc) while Thailand, despite not having F1, they focused on developing local scene & even local corporations financially backed them (just watch Thailand Super Series, you don't see those here, although they visit Sepang every year).
This piece from my friend (one of local racer/organizer/promoter, he's been in it for 20 years) is from 8 years ago & still scarily holds up today.

If you see anyone opposes F1 coming back here, now you know why.

Yeah, besides, in those days, stock Civics were quite affordable compared to now. And you can swap parts easily. Nowadays even changing wheel sizes you will probably get an electronic error message. BTW, Is there a Myvi or Axia race series?