The ICIS manual (included in the gold manual, people...) doesn't explain *why*, it just tells you that the bugs can shoot down the recon Excaliburs (six of them) despite their cloaks.
And no, the cloaking device is *not* a regular feature on the Excalibur -- the variant that hit Kilrah in WC3 mounted it, as does the RF-103... but the F-103C and F-103D (WCIV and SO) don't.
The only time you can see the outlines of cloaked ships in WCIV is when you're flying a Lance.