"more activity" why singular?

In this particular case, ‘activity’ designates a body of actions undertaken by somebody and is uncountable. Uncountable nouns refer to actions/objects collectively. A few examples of phrases with the same characteristics: police work/activity, physical exercise, manual work, good advice, etc. We can use determinants such as more/less, intense/mild, better/worse in relation to such words, but we cannot exactly say how much or how many of ‘it’ somebody has done/undergone, etc. ‘Activity’ may be countable or uncountable, but there are many nouns that are uncountable only, whether defective of singular or of plural. According to https://dictionary.cambridge.org/dictionary/english/activity (visited on 05/11/2020), activity means: def. B2 [uncountable] “the situation in which a lot of things are happening or people are moving around: […] Economists are concerned by the low level of economic activity”; def. B2 [countable & uncountable] “the work of a group or organization to achieve an aim: He was found guilty of terrorist activity”.

Simply put, Activity can be countable or uncountable.

In its uncountable form, it encompasses all types of activity; activity in general:

"The building site was a hive of activity."

In its countable form, it refers to separate/individual activities:

"The carpenters, builders, plasterers and roofers were busily engaged in their activities."

Collins Dictionary: Activity/activity

1. uncountable noun

Activity is a situation in which a lot of things are happening or being done.

...an extraordinary level of activity in the government bonds market.

...the electrical activity of the brain. [+ of]

2. countable noun

An activity is something that you spend time doing.

You can take part in activities from canoeing to bird watching.

When an uncountable noun is qualified by an adjective or adjunct, it may accept the indefinite article in its meaning of "an example of".

Consider:

"Ask John, he has more knowledge of the area [than me]"

And thus we have:

[It] can fuel more illegal activity (uncountable) [than I have mentioned]