There have been several romanization schemes for the Chinese languages over the years, and in recent decades we seem to have settled in Pinyin as the system of choice. There doesn't seem to be a comparable standard system for Arabic. Why not?
The problem isn't no standard—the problem is too many standards!
There are at least a dozen competing systems for romanization of Arabic, all mutually incompatible, all used for different purposes. For example, ISO 233 (specifically ISO 233-2 now) is backed by the International Standards Organization, which is about the best endorsement you can get, but it requires various diacritics that aren't on most keyboards. ArabTeX is standard for typesetting and uses pure ASCII, but gets unwieldy to read on its own. And so on and so forth.
Pinyin has three things that none of these Arabic standards have:
- It doesn't require any "exotic" letters (*), making it easy to adapt for ASCII and appropriate for publication at the same time
- It has the endorsement of the Chinese government, and the majority of all Chinese-language writings are written/published in China
- It fills a niche where the native writing system is severely lacking (unambiguously showing the pronunciation of rare or unusual words)
Meanwhile, there is no central government that the majority of Arabic-speakers in the world listen to, the native Arabic alphabet shows pronunciations already, and I've never seen a system that avoids "exotic" letters, looks good in print, and represents the whole phonology unambiguously.
(*) "Exotic" in the world of Unicode tends to mean "what Western Europeans are unfamiliar with". So ħ is significantly more "exotic" than ü, and significantly less likely to be supported by keyboards, typesetting systems, text processing systems, and so on.