We see from the medrash that until Yaakov came and began working for him, he had no sons to take care of his sheep. Nor could he afford to hire a shepherd in order to take care of his sheep. This is unlike Yisro who was being blackballed by the local shepherds so that his daughters were forced to take care of his sheep (which required all seven of them). Lavan had so few sheep that his youngest daughter was able to take care of them. Thus, the pasuk emphasizes that Rachel (was forced) to be the shepherdess.
Before Jacob's arrival Laban's flocks were scanty, as they had always
decreased through pestilence (Pirḳe R.El. xxxvi.). When Jacob had
completed his seven years of service, Laban assembled his countrymen
and consulted them as to the best means to retain him; "for," said he,
"ye know that formerly we had a scarcity of water, and it is only
through this righteous man that we are now blessed with an abundance
of it." His countrymen advised him to substitute Leah for Rachel
(Targ. pseudo-Jonathan and Yerushalmi to Gen. xxix. 22; Gen. R. lxx.
Additionally, we see that in Vayeitzei 31:1 that now Lavan's sons appear and complain abot him. There are meforshim who say that since they had not been born before he came, they only saw the wealth that he had gained from Hashem and thought that their father had always been rich.
Of course part of that was because Lavan kept complaining and refusing to acknowledge the good fortune that Yaakov had brought him, while at the same time scheming to prevent him from leaving in order that the blessing would continue.
Art Scroll Chumash Vayeitzei 29:9
כִּי רֹעָה הִוא For she was a shepherdess. Rachel tended the flocks
alone, for Lavan had no other shepherd. Leah did not share this chore,
either because the sun might have been harmful to her weak eyes [see
verse 17], or because she was older, and Lavan was afraid to let her
mingle with the shepherds. Rachel however, was still too young to
arouse the interest of the shepherd boys. (Ramban)
The verse implies that Rachel came leading all of Lavan's sheep, which
indicates that the flock was small. Indeed, Lavan became prosperous
only after Jabob began working for him [see 30:30]. Even so, Rachel
must have been a skilled shepherdess to tend single-handedly to even a
small flock at such a young age (Ohr HaChaim).
For the little that you had before me has increased in multitude, and
the Lord blessed you upon my arrival; but now, when will I, too,
provide [something] for my household?