Because if you do some research then you find that Dragon Ball GT Goku is dangerously stronger, and even in his BASE FORM, he's stronger than even Beerus, the God of destruction.
After Goku unlocks the form of Super Saiyan God, he fights Beerus and is soundly defeated. This eased Beerus' worries greatly, though it seems he should have been worried about an entirely different, and totally surprising, Dragon Ball hero instead.
Goku admits that he was holding back at first only using 80% of his real power which would put Goku's battle power at 4.8 Quadrillion.
This is a phenomenal display of strength, but Goku learns that his temperamental new transformation still isn't enough to beat Jiren on his own. Ultra Instinct is considerably stronger than Dragon Ball GT's Super Saiyan 4 transformation.
Not only does this show that GT Goku is stronger (He fought people STRONGER than Super Buu in base form.) Not to mention that if it's true that Goku absorbed ALL of Super Saiyan god's power into his base form, then this ALSO shows that Super Saiyan god's power is only 8X higher than Super Saiyan 3.
How old is Goku in Dragon Ball GT? Dragon Ball GT opens five years later, shifting Goku to physically 42 but chronologically 49 years old. Emperor Pilaf steps in to complicate things even further after reverting him back to an 11-year-old while retaining his adult mentality.
In the anime-only sequel series Dragon Ball GT, Goku is transformed back into a child by an accidental wish made by his old enemy Pilaf using the Black Star Dragon Balls while Pilaf was about to wish to take over the world.
He didn't get physically weaker, no, but his stamina did decrease significantly.
Goku's training between the Baby and S17 sagas got him to surpass Super Baby Vegeta 2 in SSJ, an increase that makes him at least 64x stronger over a single year timespan (which is the length of time between Baby and S17). Multiply that by 100 and Timeskip Base GT Goku would be 6400x stronger than Omega Shenron.
He didn't get any weaker. The only difference was stamina and he only felt that in his SSJ3 form. This is stated in the GT perfect files. He didn't drop in power at all.
Immortality: As shown in Dragon Ball GT: Hero's Legacy, Goku cannot age and will presumably live forever. It is unknown if this immortality extends to him never being killed, but he certainly has eternal youth.
In a word, Saitama is the most vital being in his universe and would probably have no trouble beating Beerus.
That said, it really does seem like Zeno will stay the strongest god in Dragon Ball for the foreseeable future. His level of power is so incomprehensible that it seems highly unlikely Goku will ever fight Zeno, let alone beat him.
Jiren's wish was to resurrect his fallen mentor!
Unable to receive recognition from Gicchin as his successor before he passed away, Jiren wanted to resurrect him to show him the incredible power he had gained.
Perfected Ultra Instinct is known by other names such as Mastered and Completed Ultra Instinct, and is still the apex of Goku's power. Not only did Goku beat Jiren in this form, but he also defeated Moro as well.
1 Mastered Ultra-Instinct
Combat-wise, it was a simple fact that he could fight much better, and allow his combat skills to kick in all while his body dodges attacks. This form is Goku's strongest form, and any opponent who forces Goku to this state will have much to deal with.
When Xeno Goku appears in the Dark Demon Realm Saga of Dark Demon Realm Mission he is said to be "the strongest warrior", suggesting that he is more powerful than Demon God Demigra, who had appeared before him. The character chart describes him as the strongest Saiyan.
However, both are considered canon within the anime. While they should be seen as a continuation of Goku's story, it is not necessary to watch either Dragon Ball Super or Dragon Ball GT. There are some great moments in these shows but they aren't required viewing for Dragon Ball fans.
Toriyama did, however, come up with the Dragon Ball GT name, which stands for "Grand Tour", in reference to the series having the characters travel through the universe, and designed the appearances of the main cast. Toriyama himself referred to GT as a "side story of the original Dragon Ball".
While it's easy to assume that this version of Goku is the one that exists in the GT timeline, that's actually not the case. Xeno Goku's history is similar to Dragon Ball GT's Goku in almost every respect, save for a few core events.