I heard that the letter "g" usually takes the sound of the letter "j" before "e" and "i". This happens in both English and in Spanish. The diagraph "gu" takes the sound of the letter "g" before "e" and "i" in both English and Spanish. I cannot understand that if the English verb "sing" conjugates to "singing" and not "singuing" for the present participle, then how come the Spanish verb "llegar" conjugates to "llegué" for the first person singular simple past?
There are no pronunciation rules in the English language, just pronunciation hints at best.
However, Spanish has a phonemic ortography. For instance, "ge" is always pronounced in one specific way, no exceptions. Therefore, the G in *"llegé" would be pronounced like a J would be.
Generally, the spoken language influences the written language, rather than the other way around: "Llegar" is a regular verb, which means that the first person singular perfect simple past is going to be the root ("lleg-") plus "-é". But, rather than applying this rule to the written form, then pronouncing this form according to the usual rules, we apply the rule to the spoken form, and then write it down. We must insert a "u" between "lleg-" and "-é", because we would be changing the spoken form of the word otherwise.