Just the presence of actin filaments does not mean a cellular structure that contains it would be dynamic. Moreover there are different types of actin filaments (filopodia, lamellipodia etc) and they have different dynamics.
Microvilli closely resemble filopodia; both these structures also share some regulatory and structural proteins (other than actin). However, there are some differences which makes filopodia highly dynamic in comparison to microvilli (Sebé-Pedrós et al., 2013; Sauvanet et al., 2016).
Microvilli, though not as dynamic as filopodia, are not static structures. They can grow and retract and there are regulatory mechanisms that govern these processes (Sauvanet et al., 2016).
REGULATION AND DYNAMICS OF MICROVILLI AND MICROVILLAR COMPONENTS
Microvilli on cultured cells are dynamic structures with three phases
in their life cycle: (a) the generation of microvilli growing at 5
nm/s, (b) a steady state of ∼5 min, and (c) a retraction phase with a
rate of 1.2 nm/s (Gorelik et al. 2003). This leads to an average life
cycle of 12.1 ± 5.6 min. The turnover of microvillar proteins has been
recognized for a long time (Stidwill et al. 1984). The F-actin core in
the microvilli of Caco-2 cells at steady state treadmills, with a rate
of monomer addition at the barbed end of approximately 1.5–3.0
s−1 (Loomis et al. 2003, Tyska & Mooseker 2002),
corresponding to approximately 0.2 μm/min. The protocadherin
intermicrovillar link stabilizes microvilli of intestinal epithelial
cells, resulting in a prolonged steady state (Crawley et al. 2014).
The actin filaments in the core of stereocilia treadmill more slowly,
at 0.002–0.04 subunits s−1 (Rzadzinska et al. 2004) or
hardly at all (Zhang et al. 2012).