They could and they did, That's where Spartan IIIs came in, Unless you were referring to the Spartan IIs in which case, The project was abandoned after the initial test, Because it took many many years to train them, Cost incredible amounts of money even for one Spartan and a majority of the candidates in the program didn't even survive it to become mission-capeable soldiers.
Which is why afterwards, Colonel Ackerson started up the Spartan III project, As a way of making cheaper, More expendable Spartans, Who were simply trained, augmented, Then sent on suicide missions to help humanity. Of which they did bring in lots more candidates and made lots more Spartans, But by the time it would have taken them to train another batch, (even the shorter trained S3s took many years) The war was already over.Money.
The Spartan IIs Like John 117 cost millions to train.
The Spartan II program's funding was cut.
Spartan IIIs are cheaper and better succes rate, but their top secret traing base on Onyx was destroyed about the timeline of Halo 2-3
Spartans cost a lot of resourcesBecause The War Between The UNSC And The Covenant Has Already ENDED, And The War Between The UNSC And Rebels Has Also Ended.they were trained on reach and it destrotedthey can it more a matter of the time it takes to create them, which due to the rate the covies are closing in on them is rather limited. Also the cost of spartans is not inconsiderably Short answer: Time and Funding.
Long answer: The SPARTAN II program required training children from a young age (Master Chief was six at conscription) and putting them through near torturous training and (very) torturous bio-augmentation. 75% of the candidates washed out of the program (read: were maimed or killed by the augmentation/training).
The SPARTAN III program was an attempt at recreating the results of S-II with less of a time and monetary investment. S-IIIs were still expensive and nowhere near as effective.
As the Covenant closed in on the inner human colonies and Earth, the SPARTAN programs were scrapped in order to divert resources into more immediate defenses of Earth and her colonies.